System And Method For Fast Acquisition Of Ultra Wideband Signals

ABSTRACT

An ultra wideband system employing a threshold to detect signal quality during acquisition wherein the threshold is adjusted based on signal characteristics such as packet traffic rate, packet loss rate, and packet loss fraction. In one embodiment, the threshold is adjusted by adjusting the gain of a variable gain stage ahead of the threshold. In another embodiment, gain and threshold are adjusted in a coordinated manner wherein gain is adjusted for low signal levels and threshold is adjusted for high signal levels. In one embodiment, packet traffic rate is evaluated over an interval based on maximum packet length, number of monitor packets, and inter-packet delay. In a further embodiment, multiple ramp builders are operated in parallel at multiple code offset values to generate signal statistics to compare with the threshold. Embodiments are disclosed wherein the thresholds are adaptively adjusted based on signal performance characteristics or the multipath environment.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application titled “Systemand Method for Fast Acquisition of Ultra Wideband Signals”, Ser. No.10/955,118 118 filed Sep. 30, 2004 by Fisher et al., which is acontinuation in part of application titled: “Methods and SystemsAcquiring Impulse Signals,” Ser. No. 10/713,615, filed Nov. 14, 2003 byBrethour et al., which claims the benefit of provisional applicationtitled: “Methods and Systems for Acquiring Impulse Signals,” Ser. No.60/426,949 filed Nov. 15, 2002 by Brethour et al., and provisionalapplication titled “Method And Apparatus For Ultra Wideband SignalingAnd Modulation,” Ser. No. 60/426,857 filed Nov. 15, 2002 by Brethour etal.; the present application is also a continuation in part ofapplication titled: “System And Method For Processing Signals In UWBCommunications”, Ser. No. 10/712,269, filed on 14 Nov. 2003 by Brethouret al., which claims the benefit of provisional application titled:“Method And Apparatus For Ultra Wideband Signaling And Modulation,” Ser.No. 60/426,857 filed Nov. 15, 2002 by Brethour et al.; the presentapplication further claims benefit of provisional application titled:“Selective Auto Gain Control with Auto Threshold,” Ser. No. 60/506,761filed Sep. 30, 2003 by Fisher et al.; all of which are incorporatedherein by reference.

GOVERNMENT RIGHTS

The US Government has a paid-up license in this invention and the rightin limited circumstances to require the patent owner to license otherson reasonable terms as provided for by the terms of contractDAAB07-03-D-213 awarded by the U.S. Army CECOM Night Visions Lab, FortBelvoir, Va.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of communications, and moreparticularly, the present invention relates to a mechanism for acquiringan ultra wideband signal.

2. Background Art

An impulse radio system typically includes an impulse radio transmitterfor transmitting an impulse signal and an impulse radio receiver forreceiving the impulse signal. An exemplary impulse signal includes atrain of impulse signal frames each including one or more impulses. Thetransmitter can pulse position modulate the impulses within the impulsesignal frames based on a modulating signal, and then transmit theimpulse signal frames to the receiver.

The impulse radio receiver receives the impulse signal frames andassociated received impulses transmitted by the impulse radiotransmitter. In one known application, the impulse receiver coherentlysamples the received impulses to produce impulse samples. The receivercan use such impulse samples for subsequent signal processing relatingto radar, position-locating, and communication applications, forexample. However, before the impulse receiver can coherently sample thereceived impulses, it is necessary for the impulse receiver to determinea frame timing associated with the received impulses. That is, theimpulse radio receiver must achieve frame synchronization (also referredto as proper frame alignment). For example, it is useful for the impulsereceiver to determine when each received impulse signal frame beginsand/or ends. Therefore, in an impulse radio capable of receiving animpulse signal including a train of impulse signal frames, there is aneed to determine received impulse signal frame timing, such as a timewhen each impulse signal frame begins and/or ends.

Additionally, if the received impulse signal is coded (e.g., pulseposition modulated based on a pseudo-random (PN) code), then the impulseradio receive must achieve code synchronization before it can coherentlysample the received signal. Thus, there is also a need to provide codesynchronization in an impulse radio capable of receiving an impulsesignal including a train of code modulated impulse signals.

The impulse radio transmitter can transmit source information (i.e.,digital data) to the impulse radio receiver. For example, the impulsetransmitter uses the source information to pulse position modulate theimpulses within the impulse signal frames, thereby producing informationbearing symbols. The transmitter transmits the symbols (i.e., theimpulse signal frames including the pulse position modulated impulses)to the impulse receiver. It is likely that the transmitter also codesthe symbols as mentioned above prior to transmitting the impulse signalframes.

The impulse radio receiver receives the symbols transmitted by theimpulse transmitter. Before the impulse receiver can demodulate thereceived symbols to recover the source information therein, the impulsereceiver needs to recover a symbol timing associated with the receivedsymbols. For example, the receiver needs to determine when each receivedsymbol begins and/or ends. Once the receiver recovers such symboltiming, then the receiver can demodulate the received symbols to recoverthe data therein. Therefore, in an impulse radio capable of receivingsymbols, there is a need to determine (or recover) received symboltiming, thereby enabling the impulse radio receiver to demodulate thesymbols.

The impulse radio transmitter can pulse position modulate the impulseswithin the impulse signal frames based on different types of codesequences (codes), to produce a coded impulse signal. One type of codeis a PN code used to channelize the impulse signal and/or combatrelatively narrowband interference signals. These codes are relativelylong (e.g., a code length of 1024, 2048 or 4096) for at least tworeasons: first, so energy is spread across the frequency spectrum; andsecond, so a relatively large number of independent communicationchannels are provided.

In order to demodulate the coded impulse signal, the impulse radioreceiver must be code synchronized with the impulse signal transmitter.Accordingly, there is a need to code synchronize the impulse radioreceiver with the impulse radio transmitter.

It is typically beneficial to accomplish necessary requirements in fastand efficient manners that utilize reduced amounts of hardware tothereby increase throughput and/or reduce hardware costs. Morespecifically, it would be beneficial to satisfy each of the abovediscussed needs in fast and efficient manners that utilize reducedamounts of hardware. For example, it would be beneficial to achieveproper frame alignment in a fast and an efficient manner that utilizesreduced amounts of hardware. Further, it would be beneficial to recoverreceived symbol timing in a fast and efficient manner that utilizesreduced amounts of hardware. Additionally, it would be beneficial tocode synchronize an impulse radio receiver with the code of an impulseradio transmitter in a fast and efficient manner that utilizes reducedamounts of hardware. Still further, it would be beneficial if the samehardware could be used (or reused) to satisfy as many of the above needsas possible.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention is an ultra wideband system employing athreshold to detect signal quality during acquisition wherein thethreshold is adjusted based on signal characteristics such as packettraffic rate. In one embodiment, the threshold is adjusted by adjustingthe gain of a variable gain stage ahead of the threshold. In anotherembodiment, gain and threshold are adjusted in a coordinated mannerwherein gain is adjusted for low signal levels and threshold is adjustedfor high signal levels.

In one embodiment, packet traffic rate is evaluated over an intervalbased on maximum packet length, number of monitor packets, andinter-packet delay.

In a further embodiment, multiple ramp builders are operated in parallelat multiple code offset values to generate signal statistics to comparewith the threshold. The signal statistics may include:

Number Threshold Equation 1 N * MaxR/MA_meanV > C 2 MaxR/NextR > C 3Logic AND or OR of equation 1 and 2 (via the Master Sequencer) 4 N *MaxR/MaxV > C 5 N * (MaxR/MA_meanV) * (MinV/MaxV) > C 6 N *(MaxR/MA_meanV) * [FORCE_ZERO(MinV − MA_MinV)/FORCE_ONE(MaxV −MA_MinV)] > C 7 N * (MaxR − NextR)/MA_meanV > C 8 N * [(MaxR −NextR)/MA_meanV] * (MinV/MaxV) > C

In a further embodiment, frame offset space is scanned to determine themaximum signal strength and thresholds are set below the maximum signalstrength.

In a further embodiment, frame offset space is scanned to determine anoise floor and a maximum signal strength wherein the threshold value isestablished between the noise floor and the maximum signal strength.

In a further embodiment having multiple stages of acquisition, thethresholds for each stage are adaptively adjusted according toperformance criteria wherein the multiple stages include at least one ofthe following:

Initial Detection—First detection of a possible signal.

Initial Tracking—Initial closed loop tacking of the signal

Sig-Opt—Signal Optimization, refinement of tracking loop lock point

Quick Check—Verification of the first detection.

Lock Check—verification of tracking loop lock

Long Code (Ratchet Code) Detection—Detection of long length code

Detect Delimiter—Detect end of acquisition header

In a further embodiment having multiple stages of acquisition, thethresholds for each stage are adaptively adjusted according toperformance criteria, the performance criteria include at least one ofthe following:

false alarm rate,

packet acquire rate, or

packet loss rate

In a further embodiment, packet sent information is encoded intransmitted packets and the packet sent information is used to derivethe packet loss rate for use in setting threshold levels.

In a further embodiment, wherein a system may receive a giventransmitter multiple times on a known schedule, the threshold parametersassociated with a given transmitter may be stored in memory to beretrieved from memory to search for the given transmitter.

In a further embodiment, thresholds may be optimized by a firsttransceiver and the optimization information sent to a secondtransceiver to aid in the reception of the first transceiver by thesecond transceiver.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1A illustrates a representative Gaussian Monocycle waveform in thetime domain, which is the first derivative of a Gaussian pulse;

FIG. 1B illustrates the frequency domain amplitude of the GaussianMonocycle of FIG. 1A;

FIG. 1C represents the second derivative of a Gaussian pulse;

FIG. 1D represents the third derivative of the Gaussian pulse;

FIG. 1E represents the Correlator Output vs. the Relative Delay of ameasured pulse signal;

FIG. 1F depicts the frequency domain amplitude of the Gaussian family ofthe Gaussian Pulse and the first, second, and third derivative;

FIG. 2A illustrates a pulse train comprising pulses as in FIG. 1A;

FIG. 2B illustrates the frequency domain amplitude of the waveform ofFIG. 2A;

FIG. 2C illustrates the pulse train spectrum;

FIG. 2D is a plot of the Frequency vs. Energy;

FIG. 3 illustrates the cross-correlation of two codes graphically asCoincidences vs. Time Offset;

FIGS. 4A-4E illustrate five modulation techniques to include: Early-LateModulation; One of Many Modulation; Flip Modulation; Quad FlipModulation; and Vector Modulation;

FIG. 5A illustrates representative signals of an interfering signal, acoded received pulse train and a coded reference pulse train;

FIG. 5B depicts a typical geometrical configuration giving rise tomultipath received signals;

FIG. 5C illustrates exemplary multipath signals in the time domain;

FIG. 5D represents a signal plot of an idealized UWB received pulse withno multipath;

FIG. 5E represents a signal plot of an idealized UWB received pulse inmoderate multipath;

FIG. 5F represents a signal plot of an idealized UWB received pulse insevere multipath;

FIG. 5G illustrates the Rayleigh fading curve associated withnon-impulse radio transmissions in a multipath environment;

FIG. 5H illustrates a plurality of multipaths with a plurality ofreflectors from a transmitter to a receiver;

FIG. 5I graphically represents signal strength as volts vs. time in adirect path and multipath environment;

FIG. 6 is an illustration of an example general purpose architecture foran impulse radio;

FIG. 7 is a more detailed block diagram of the impulse radio of FIG. 6;

FIG. 8A is an illustration of a transmitted impulse transmitted by aremote impulse radio and received by an impulse radio antenna;

FIG. 8B is an illustration of an example impulse response of an impulseradio receiver front-end;

FIG. 9 is a block diagram of an example (IJ) correlator pair arrangementcorresponding to a sampling channel in the impulse radio of FIG. 7;

FIG. 10A is an example timing waveform representing a correlatorsampling control signal in the impulse radio of FIG. 7, and in the (IJ)correlator pair arrangement of FIG. 9;

FIG. 10B is an example timing waveform representing a first samplingsignal derived by a sampling pulse generator of FIG. 9;

FIG. 10C is an example timing waveform representing a second samplingsignal produced by a delay of FIG. 9;

FIG. 11 is a block diagram of an exemplary lock loop used for tracking areceive impulse signal;

FIGS. 12A and 12B illustrate exemplary packets 1202 transmitted by animpulse radio transmitter, according to embodiment of the presentinvention;

FIG. 13 illustrates an example timing relationship between a portion ofa received impulse signal including received header frames and areceiver sample timeline initially established by the impulse radio tosample the received impulse signal;

FIG. 14 shows an exemplary acquisition code sequence and itscorresponding delimiter;

FIG. 15 shows a high level flow diagram illustrating a method foracquiring a pulse position modulated (ppm) impulse signal, according toan embodiment of the present invention;

FIG. 16 a illustrates additional details of one of the steps shown inFIG. 15, according to an embodiment of the present invention;

FIG. 16 b depicts discriminants;

FIG. 16 c depicts a quantity threshold comparison flowchart;

FIGS. 17 and 18 illustrate time-lines that are useful for explainingspecific parallel steps of discussed in FIG. 16;

FIG. 19 shows an exemplary packet that is useful for explaining theambiguity that results from the integration length used by an impulseradio receiver being greater than the number of ramp builders in theimpulse radio receiver;

FIG. 20 illustrates an exemplary packet and relative times over whichramp values can be generated, is useful for explaining how a ratchetcodes of the present invention can be used to resolve a four (4) wayambiguity that results when a receiver uses four (4) ramp buildersduring signal acquisition, and the integration length is sixteen (16),and is useful for explaining the steps of FIG. 21;

FIGS. 21 and 22 illustrate additional details of one of the steps of themethod shown in FIG. 15, according to an embodiment of the presentinvention;

FIG. 23 illustrates a table that is useful for showing how ratchet codescan be generated based on a length four (4) short acquisition code,according to an embodiment of the present invention;

FIG. 24 shows an exemplary portion of an impulse radio receiveraccording to an embodiment of the present invention;

FIG. 25 illustrates an exemplary portion of an impulse radio receiverthat is used to produce back ramp values, according to an embodiment ofthe present invention;

FIG. 26 illustrates an exemplary frame, which is useful for showing howmultiple states can be represented according to various embodiments ofthe present invention;

FIG. 27 shows an exemplary portion of an impulse radio receiveraccording to an embodiment of the present invention;

FIG. 28 illustrates a table that is useful for showing how ratchet codescan be generated based on a length sixteen (16) short acquisition code,according to an embodiment of the present invention;

FIG. 29 illustrates a portion of a frame, which is useful for showinghow impulses are deliberately jittering in an embodiment of the presentinvention;

FIG. 30 shows an exemplary portion of an impulse radio receiveraccording to an embodiment of the present invention; and

FIG. 31 is an example computer system environment in which the presentinvention can operate.

FIG. 32 shows a gain plot for a variable gain system wherein both gainand threshold are varied in a coordinated manner, in accordance with thepresent invention.

FIG. 33 is a functional flow diagram for a receiver employing a variablegain and threshold system in accordance with the present invention.

FIG. 34 is a functional flow diagram illustrating an exemplarymulti-stage acquisition process in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION Table of Contents I. Impulse RadioBasics

A. Overview

B. Waveforms

C. Pulse Trains

D. Coding

E. Modulation

F. Reception and Demodulation

G. Interference Resistance

H. Processing Gain

I. Capacity

J. Multipath and Propagation

K. Distance Measurement and Positioning

L. Power Control

II. Exemplary General Purpose Architectural Embodiment for an ImpulseRadio Transceiver

A. Overview

B. RF Sampling Subsystem

C. Timing Subsystem

D. Control Subsystem

E. Baseband Processor

F. Paired Correlators

G. Lock Loop

III. Preferred Embodiments

A. Terminology

B. Exemplary Packets

C. Problem Description

D. Overview of Solution to Problem

E. Packet Protocol

F. Frame Alignment (First Stage Acquisition)

G. Threshold Determination

H. Tracking

I. Back Ramps

J. Detect Beginning of Data Payload (Second Stage Acquisition)

K. Ratchet Codes

L. Generating Ratchet Codes

M. Exemplary Impulse Radio Receiver Subsystem

-   -   1. Operation During First Stage Acquisition    -   2. Tracking and Back Ramps    -   3. Operation During Second Stage Acquisition    -   4. Frame Format and Additional Receiver Embodiments for Use with        Longer Short Acquisition Codes and Ratchet Codes    -   5. IJ or IQ Correlator Pairs and Ramp Builder Pairs

N. Radio Command Channel

O. Jittering Impulse Positions

P. Acquisition Times

-   -   1. First Stage Acquisition Time    -   2. Tracking Time    -   3. Second Stage Acquisition Time    -   4. Time Required for Complete Acquisition    -   5. Time Required in a Typical Conventional System    -   6. Comparison

Q. Hardware and Software Implementations

R. Automatic Acquisition Threshold

S. Gain Control

T. Automatic Multi-Stage Threshold Adjustment

IV. Conclusion I. Impulse Radio Basics

The present invention builds upon existing impulse radio techniques.Accordingly, an overview of impulse radio basics is provided prior to adiscussion of the specific embodiments of the present invention. Thissection is directed to technology basics and provides the reader with anintroduction to impulse radio concepts, as well as other relevantaspects of communications theory. This section includes subsectionsrelating to waveforms, pulse trains, coding for energy smoothing andchannelization, modulation, reception and demodulation, interferenceresistance, processing gain, capacity, multipath and propagation,distance measurement, and qualitative and quantitative characteristicsof these concepts. It should be understood that this section is providedto assist the reader with understanding the present invention, andshould not be used to limit the scope of the present invention.

A. Overview

Ultra Wideband is an emerging RF technology with significant benefits incommunications, radar, positioning and sensing applications. In 2002,the Federal Communications Commission (FCC) recognized these potentialbenefits to the consumer and issued the first rulemaking enabling thecommercial sale and use of products based on Ultra Wideband technologyin the United States of America. The FCC adopted a definition of UltraWideband to be a signal that occupies a fractional bandwidth of at least0.25, or 0.5 GHz bandwidth at any center frequency. The 0.25 fractionalbandwidth is more precisely defined as:

${{F\; B\; W} = \frac{2\left( {f_{h} - f_{l}} \right)}{f_{h} + f_{l}}},$

where FBW is the fractional bandwidth, fh is the upper band edge and flis the lower band edge, the band edges being defined as the 10 dB downpoint in spectral density.

There are many approaches to UWB including impulse radio, directsequence CDMA, ultra wideband noise radio, direct modulation of ultrahigh-speed data, and other methods. The present invention has its originin ultra wideband impulse radio and will have significant applicationthere as well, but it has potential benefit and application beyondimpulse radio to other forms of ultra wideband and beyond ultra widebandto conventional radio systems as well. Nonetheless, it is useful todescribe the invention in relation to impulse radio to understand thebasics and then expand the description to the extensions of thetechnology.

The following is an overview of impulse radio as an aid in understandingthe benefits of the present invention.

Impulse radio has been described in a series of patents, including U.S.Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057(issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990),and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton.A second generation of impulse radio patents includes U.S. Pat. No.5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov.11, 1997), U.S. Pat. No. 5,764,696 (issued Jun. 9, 1998), U.S. Pat. No.5,832,035 (issued Nov. 3, 1998), and U.S. Pat. No. 5,969,663 (issuedOct. 19, 1999) to Fullerton et al., and U.S. Pat. No. 5,812,081 (issuedSep. 22, 1998), and U.S. Pat. No. 5,952,956 (issued Sep. 14, 1999) toFullerton, which are incorporated herein by reference.

Uses of impulse radio systems are described in U.S. Pat. No. 6,177,903(issued Jan. 23, 2001) titled, “System and Method for IntrusionDetection using a Time Domain Radar Array” and U.S. Pat. No. 6,218,979(issued Apr. 17, 2001) titled “Wide Area Time Domain Radar Array”, whichare incorporated herein by reference.

Acquisition approaches involving acquisition thresholds are described inU.S. Pat. No. 5,832,035, titled “Fast Locking Mechanism for ChannelizedUltrawide-Band Communications,” issued Nov. 3, 1998 to Fullerton, whichwas incorporated by reference above, and in U.S. Pat. No. 6,556,621,titled “System and Method for Fast Acquisition of Ultra WidebandSignals,” issued Apr. 29, 2003 to Richards et al., which is incorporatedherein by reference.

This section provides an overview of impulse radio technology andrelevant aspects of communications theory. It is provided to assist thereader with understanding the present invention and should not be usedto limit the scope of the present invention. It should be understoodthat the terminology ‘impulse radio’ is used primarily for historicalconvenience and that the terminology can be generally interchanged withthe terminology ‘impulse communications system, ultra-wideband system,or ultra-wideband communication systems’. Furthermore, it should beunderstood that the described impulse radio technology is generallyapplicable to various other impulse system applications including butnot limited to impulse radar systems and impulse positioning systems.Accordingly, the terminology ‘impulse radio’ can be generallyinterchanged with the terminology ‘impulse transmission system andimpulse reception system.’

Impulse radio refers to a radio system based on short, wide bandwidthpulses. An ideal impulse radio waveform is a short Gaussian monocycle.As the name suggests, this waveform attempts to approach one cycle ofradio frequency (RF) energy at a desired center frequency. Due toimplementation and other spectral limitations, this waveform may bealtered significantly in practice for a given application. Manywaveforms having very broad, or wide, spectral bandwidth approximate aGaussian shape to a useful degree.

Impulse radio can use many types of modulation, including amplitudemodulation, phase modulation, frequency modulation (including frequencyshape and wave shape modulation), time-shift modulation (also referredto as pulse-position modulation or pulse-interval modulation) and M-aryversions of these. In this document, the time-shift modulation method isoften used as an illustrative example. However, someone skilled in theart will recognize that alternative modulation approaches may, in someinstances, be used instead of or in combination with the time-shiftmodulation approach.

In impulse radio communications, inter-pulse spacing may be heldconstant or may be varied on a pulse-by-pulse basis by information, acode, or both. Generally, conventional spread spectrum systems employcodes to spread the normally narrow band information signal over arelatively wide band of frequencies. A conventional spread spectrumreceiver correlates these signals to retrieve the original informationsignal. In impulse radio communications, codes are not typically usedfor energy spreading because the monocycle pulses themselves have aninherently wide bandwidth. Codes are more commonly used forchannelization, energy smoothing in the frequency domain, resistance tointerference, and reducing the interference potential to nearbyreceivers. Such codes are commonly referred to as time-hopping codes orpseudo-noise (PN) codes since their use typically causes inter-pulsespacing to have a seemingly random nature. PN codes may be generated bytechniques other than pseudorandom code generation. Additionally, pulsetrains having constant, or uniform, pulse spacing are commonly referredto as uncoded pulse trains. A pulse train with uniform pulse spacing,however, may be described by a code that specifies non-temporal, i.e.,non-time related, pulse characteristics.

In impulse radio communications utilizing time-shift modulation,information comprising one or more bits of data typically time-positionmodulates a sequence of pulses. This yields a modulated, coded timingsignal that comprises a train of pulses from which a typical impulseradio receiver employing the same code may demodulate and, if necessary,coherently integrate pulses to recover the transmitted information.

The impulse radio receiver is typically a direct conversion receiverwith a cross correlator front-end that coherently converts anelectromagnetic pulse train of monocycle pulses to a baseband signal ina single stage. The baseband signal is the basic information signal forthe impulse radio communications system. A subcarrier may also beincluded with the baseband signal to reduce the effects of amplifierdrift and low frequency noise. Typically, the subcarrier alternatelyreverses modulation according to a known pattern at a rate faster thanthe data rate. This same pattern is used to reverse the process andrestore the original data pattern just before detection. This methodpermits alternating current (AC) coupling of stages, or equivalentsignal processing, to eliminate direct current (DC) drift and errorsfrom the detection process. This method is described in more detail inU.S. Pat. No. 5,677,927 to Fullerton et al.

B. Waveforms

Impulse transmission systems are based on short, wide band pulses.Different pulse waveforms, or pulse types, may be employed toaccommodate requirements of various applications. Typical ideal pulsetypes used in analysis include a Gaussian pulse doublet (also referredto as a Gaussian monocycle), pulse triplet, and pulse quadlet asdepicted in FIGS. 1A through 1D. An actual received waveform thatclosely resembles the theoretical pulse quadlet is shown in FIG. 1E. Apulse type may also be a wavelet set produced by combining two or morepulse waveforms (e.g., a doublet/triplet wavelet set), or families oforthogonal wavelets. Additional pulse designs include chirped pulses andpulses with multiple zero crossings, or bursts of cycles. Thesedifferent pulse types may be produced by methods described in the patentdocuments referenced above or by other methods understood by one skilledin the art.

For analysis purposes, it is convenient to model pulse waveforms in anideal manner. For example, the transmitted waveform produced bysupplying a step function into an ultra-wideband antenna may be modeledas a Gaussian monocycle. A Gaussian monocycle (normalized to a peakvalue of 1) may be described by:

${f_{mono}(t)} = {\sqrt{e}\left( \frac{t}{\sigma} \right)^{\frac{- t^{2}}{2\sigma^{2}}}}$

where σ is a time scaling parameter, t is time, and e is the naturallogarithm base.

FIG. 1F shows the power spectral density of the Gaussian pulse, doublet,triplet, and quadlet normalized to a peak density of 1. The normalizeddoublet (monocycle) is as follows:

F _(mono)(f)=j(2π)√{square root over (e)}σfe ^(−2(πσf)) ²

Where F_(mono)( ) is the Fourier transform of f_(mono)( ), f isfrequency, and j is the imaginary unit. The center frequency (f_(c)), orfrequency of peak spectral density, of the Gaussian monocycle is:

$f_{c} = \frac{1}{2\pi \; \sigma}$

C. Pulse Trains

Impulse transmission systems may communicate one or more data bits witha single pulse; however, typically each data bit is communicated using asequence of pulses, known as a pulse train. As described in detail inthe following example system, the impulse radio transmitter produces andoutputs a train of pulses for each bit of information. FIGS. 2A and 2Bare illustrations of the output of a typical 10 megapulses per second(Mpps) system with uncoded, unmodulated pulses, each having a width of0.5 nanoseconds (ns). FIG. 2A shows a time domain representation of thepulse train output. FIG. 2B illustrates that the result of the pulsetrain in the frequency domain is to produce a spectrum comprising a setof comb lines spaced at the frequency of the 10 Mpps pulse repetitionrate. When the full spectrum is shown, as in FIG. 2C, the envelope ofthe comb line spectrum corresponds to the curve of the single Gaussianmonocycle spectrum in FIG. 1F. For this simple uncoded case, the powerof the pulse train is spread among roughly two hundred comb lines. Eachcomb line thus has a small fraction of the total power and presents muchless of an interference problem to a receiver sharing the band. It canalso be observed from FIG. 2A that impulse transmission systems may havevery low average duty cycles, resulting in average power lower than peakpower. The duty cycle of the signal in FIG. 2A is 0.5%, based on a 0.5ns pulse duration in a 100 ns interval.

The signal of an uncoded, unmodulated pulse train may be expressed:

${s(t)} = {a{\sum\limits_{i = 1}^{n}{w\left( {{c\left( {t - {iT}_{f}} \right)},b} \right)}}}$

where i is the index of a pulse within a pulse train of n pulses, a ispulse amplitude, b is pulse type, c is a pulse width scaling parameter,w(t, b) is the normalized pulse waveform, and T_(f) is pulse repetitiontime, also referred to as frame time.

The Fourier transform of a pulse train signal over a frequency bandwidthof interest may be determined by summing the phasors of the pulses foreach code time shift, and multiplying by the Fourier transform of thepulse function:

${S(f)} = {a{{\sum\limits_{i = 1}^{n}^{{- j}\; 2\pi \; f\; \; T_{f}}}}{W(f)}}$

where S(f) is the amplitude of the spectral response at a givenfrequency, f is the frequency being analyzed, T_(f) is the relative timedelay of each pulse from the start of time period, W(f) is the Fouriertransform of the pulse, w(t,b), and n is the total number of pulses inthe pulse train.

A pulse train can also be characterized by its autocorrelation andcross-correlation properties. Autocorrelation properties pertain to thenumber of pulse coincidences (i.e., simultaneous arrival of pulses) thatoccur when a pulse train is correlated against an instance of itselfthat is offset in time. Of primary importance is the ratio of the numberof pulses in the pulse train to the maximum number of coincidences thatoccur for any time offset across the period of the pulse train. Thisratio is commonly referred to as the main-lobe-to-peak-side-lobe ratio,where the greater the ratio, the easier it is to acquire and track asignal.

Cross-correlation properties involve the potential for pulses from twodifferent signals simultaneously arriving, or coinciding, at a receiver.Of primary importance are the maximum and average numbers of pulsecoincidences that may occur between two pulse trains. As the number ofcoincidences increases, the propensity for data errors increases.Accordingly, pulse train cross-correlation properties are used indetermining channelization capabilities of impulse transmission systems(i.e., the ability to simultaneously operate within close proximity).

D. Coding

Specialized coding techniques can be employed to specify temporal and/ornon-temporal pulse characteristics to produce a pulse train havingcertain spectral and/or correlation properties. For example, byemploying a Pseudo-Noise (PN) code to vary inter-pulse spacing, theenergy in the uncoded comb lines presented in FIGS. 2B and 2C can bedistributed to other frequencies as depicted in FIG. 2D, therebydecreasing the peak spectral density within a bandwidth of interest.Note that the spectrum retains certain properties that depend on thespecific (temporal) PN code used. Spectral properties can be similarlyaffected by using non-temporal coding (e.g., inverting certain pulses).

Coding provides a method of establishing independent communicationchannels. Specifically, families of codes can be designed such that thenumber of pulse coincidences between pulse trains produced by any twocodes will be minimal. For example, FIG. 3 depicts cross-correlationproperties of two codes that have no more than four coincidences for anytime offset. Generally, keeping the number of pulse collisions minimalrepresents a substantial attenuation of the unwanted signal.

Coding can also be used to facilitate signal acquisition. For example,coding techniques can be used to produce pulse trains with a desirablemain-lobe-to-side-lobe ratio. In addition, coding can be used to reduceacquisition algorithm search space.

Coding methods for specifying temporal and non-temporal pulsecharacteristics are described in commonly owned, co-pending applicationstitled “A Method and Apparatus for Positioning Pulses in Time,”application Ser. No. 09/592,249, and “A Method for SpecifyingNon-Temporal Pulse Characteristics,” application Ser. No. 09/592,250,both filed Jun. 12, 2000, and both of which are incorporated herein byreference.

Typically, a code consists of a number of code elements having integeror floating-point values. A code element value may specify a singlepulse characteristic or may be subdivided into multiple components, eachspecifying a different pulse characteristic. Code element or codecomponent values typically map to a pulse characteristic value layoutthat may be fixed or non-fixed and may involve value ranges, discretevalues, or a combination of value ranges and discrete values. A valuerange layout specifies a range of values that is divided into componentsthat are each subdivided into subcomponents, which can be furthersubdivided, as desired. In contrast, a discrete value layout involvesuniformly or non-uniformly distributed discrete values. A non-fixedlayout (also referred to as a delta layout) involves delta valuesrelative to some reference value. Fixed and non-fixed layouts, andapproaches for mapping code element/component values, are described inco-owned, co-pending applications, titled “Method for Specifying PulseCharacteristics using Codes,” application Ser. No. 09/592,290 and “AMethod and Apparatus for Mapping Pulses to a Non-Fixed Layout,”application Ser. No. 09/591,691, both filed on Jun. 12, 2000, both ofwhich are incorporated herein by reference.

A fixed or non-fixed characteristic value layout may include anon-allowable region within which a pulse characteristic value isdisallowed. A method for specifying non-allowable regions is describedin co-owned U.S. Pat. No. 6,636,567 (issued Oct. 21, 2003). titled “AMethod for Specifying Non-Allowable Pulse Characteristics,” andincorporated herein by reference. A related method that conditionallypositions pulses depending on whether code elements map to non-allowableregions is described in co-owned, co-pending application, titled “AMethod and Apparatus for Positioning Pulses Using a Layout havingNon-Allowable Regions,” application Ser. No. 09/592,248 filed Jun. 12,2000, and incorporated herein by reference.

The signal of a coded pulse train can be generally expressed by:

${s_{tr}(t)} = {\sum\limits_{i}{\left( {- 1} \right)^{f_{i}}a_{i}{w\left( {{c_{i}\left( {t - T_{i}} \right)},b_{i}} \right)}}}$

where str(t) is the coded pulse train signal, i is the index of a pulsewithin the pulse train, (−1)fi, ai, bi, ci, and ω(t,bi) are the codedpolarity, pulse amplitude, pulse type, pulse width, and normalized pulsewaveform of the i'th pulse, and Ti is the coded time shift of the ithpulse. Various numerical code generation methods can be employed toproduce codes having certain correlation and spectral properties.Detailed descriptions of numerical code generation techniques areincluded in a co-owned, co-pending patent application titled “A Methodand Apparatus for Positioning Pulses in Time,” application Ser. No.09/592,248, filed Jun. 12, 2000, and incorporated herein by reference.

It may be necessary to apply predefined criteria to determine whether agenerated code, code family, or a subset of a code is acceptable for usewith a given UWB application. Criteria may include correlationproperties, spectral properties, code length, non-allowable regions,number of code family members, or other pulse characteristics. A methodfor applying predefined criteria to codes is described in co-owned U.S.Pat. No. 6,636,566 (issued Oct. 21, 2003), titled “A Method andApparatus for Specifying Pulse Characteristics using a Code thatSatisfies Predefined Criteria,” and incorporated herein by reference.

In some applications, it may be desirable to employ a combination ofcodes. Codes may be combined sequentially, nested, or sequentiallynested, and code combinations may be repeated. Sequential codecombinations typically involve switching from one code to the next afterthe occurrence of some event and may also be used to support multicastcommunications. Nested code combinations may be employed to producepulse trains having desirable correlation and spectral properties. Forexample, a designed code may be used to specify value range componentswithin a layout and a nested pseudorandom code may be used to randomlyposition pulses within the value range components. With this approach,correlation properties of the designed code are maintained since thepulse positions specified by the nested code reside within the valuerange components specified by the designed code, while the randompositioning of the pulses within the components results in particularspectral properties. A method for applying code combinations isdescribed in co-owned, co-pending application, titled “A Method andApparatus for Applying Codes Having Pre-Defined Properties,” applicationSer. No. 09/591,690, filed Jun. 12, 2000, and incorporated herein byreference.

E. Modulation

Various aspects of a pulse waveform may be modulated to conveyinformation and to further minimize structure in the resulting spectrum.Amplitude modulation, phase modulation, frequency modulation, time-shiftmodulation and M-ary versions of these were proposed in U.S. Pat. No.5,677,927 to Fullerton et al., previously incorporated by reference.Time-shift modulation can be described as shifting the position of apulse either forward or backward in time relative to a nominal coded (oruncoded) time position in response to an information signal. Thus, eachpulse in a train of pulses is typically delayed a different amount fromits respective time base clock position by an individual code delayamount plus a modulation time shift. This modulation time shift isnormally very small relative to the code shift. In a 10 Mpps system witha center frequency of 2 GHz, for example, the code may command pulseposition variations over a range of 100 ns, whereas, the informationmodulation may shift the pulse position by 150 ps. This two-state‘early-late’ form of time shift modulation is depicted in FIG. 4A.

A generalized expression for a pulse train with ‘early-late’ time-shiftmodulation over a data symbol time is:

${s_{tr}(t)} = {\sum\limits_{i = 1}^{N_{s}}{\left( {- 1} \right)^{f_{i}}a_{i}{w\left( {{c_{i}\left( {t - T_{i} - {\delta \; d_{k}}} \right)},b_{i}} \right)}}}$

where k is the index of a data symbol (e.g., bit), i is the index of apulse within the data symbol, Ns is the number of pulses per symbol,(−1)fi is a coded polarity (flipping) pattern (sequence), ai is a codedamplitude pattern, bi is a coded pulse type (shape) pattern, ci is acoded pulse width pattern, and w(t,bi) is a normalized pulse waveform ofthe ith pulse, Tj) is the coded time shift of the i'th pulse , δ is thetime shift added when the transmitted symbol is 1 (instead of 0), dk isthe data (i.e., 0 or 1) transmitted by the transmitter. In this example,the data value is held constant over the symbol interval. Similarexpressions can be derived to accommodate other proposed forms ofmodulation.

An alternative form of time-shift modulation can be described asOne-of-Many Position Modulation (OMPM). The OMPM approach, shown in FIG.4B, involves shifting a pulse to one of N possible modulation positionsabout a nominal coded (or uncoded) time position in response to aninformation signal, where N represents the number of possible states.For example, if N were four (4), two data bits of information could beconveyed. For further details regarding OMPM, see “Apparatus, System andMethod for One-of-Many Position Modulation in an Impulse RadioCommunication System,” Attorney Docket No. 1659.0860000, filed Jun. 7,2000, which is incorporated herein by reference.

An impulse radio communications system can employ flip modulationtechniques to convey information. The simplest flip modulation techniqueinvolves transmission of a pulse or an inverted (or flipped) pulse torepresent a data bit of information, as depicted in FIG. 4C. Flipmodulation techniques may also be combined with time-shift modulationtechniques to create two, four, or more different data states. One suchflip with shift modulation technique is referred to as Quadrature FlipTime Modulation (QFTM). The QFTM approach is illustrated in FIG. 4D.Flip modulation techniques are further described in patent applicationtitled “Apparatus, System and Method for Flip Modulation in an ImpulseRadio Communication System,” application Ser. No. 09/537,692, filed Mar.29, 2000, which is incorporated herein by reference.

Vector modulation techniques may also be used to convey information.Vector modulation includes the steps of generating and transmitting aseries of time-modulated pulses, each pulse delayed by one of at leastfour pre-determined time delay periods and representative of at leasttwo data bits of information, and receiving and demodulating the seriesof time-modulated pulses to estimate the data bits associated with eachpulse. Vector modulation is shown in FIG. 4E. Vector modulationtechniques are further described in patent application titled “VectorModulation System and Method for Wideband Impulse Radio Communications,”application Ser. No. 09/169,765, filed Dec. 9, 1999, which isincorporated herein by reference.

Reception and Demodulation

Impulse radio systems operating within close proximity to each other maycause mutual interference. While coding minimizes mutual interference,the probability of pulse collisions increases as the number ofcoexisting impulse radio systems rises. Additionally, various othersignals may be present that cause interference. Impulse radios canoperate in the presence of mutual interference and other interferingsignals, in part because they typically do not depend on receiving everytransmitted pulse. Except for single pulse per bit systems, impulseradio receivers perform a correlating, synchronous receiving function(at the RF level) that uses sampling and combining, or integration, ofmany pulses to recover transmitted information. Typically, 1 to 1000 ormore pulses are integrated to yield a single data bit thus diminishingthe impact of individual pulse collisions, where the number of pulsesthat must be integrated to successfully recover transmitted informationdepends on a number of variables including pulse rate, bit rate, rangeand interference levels.

G. Interference Resistance

Besides providing channelization and energy smoothing, coding makesimpulse radios highly resistant to interference by enablingdiscrimination between intended impulse transmissions and interferingtransmissions. This property is desirable since impulse radio systemsmust share the energy spectrum with conventional radio systems and withother impulse radio systems.

FIG. 5A illustrates the result of a narrow band sinusoidal interferencesignal 502 overlaying an impulse radio signal 504. At the impulse radioreceiver, the input to the cross correlation would include the narrowband signal 502 and the received ultrawide-band impulse radio signal504. The input is sampled by the cross correlator using a templatesignal 506 positioned in accordance with a code. Without coding, thecross correlation would sample the interfering signal 502 with suchregularity that the interfering signals could cause interference to theimpulse radio receiver. However, when the transmitted impulse signal iscoded and the impulse radio receiver template signal 506 is synchronizedusing the identical code, the receiver samples the interfering signalsnon-uniformly. The samples from the interfering signal add incoherently,increasing roughly according to the square root of the number of samplesintegrated. The impulse radio signal samples, however, add coherently,increasing directly according to the number of samples integrated. Thus,integrating over many pulses overcomes the impact of interference.

H. Processing Gain

Impulse radio systems have exceptional processing gain due to their widespreading bandwidth. For typical spread spectrum systems, the definitionof processing gain, which quantifies the decrease in channelinterference when wide-band communications are used, is the ratio of thebandwidth of the channel to the bit rate of the information signal. Forexample, a conventional narrow band direct sequence spread spectrumsystem with a 10 kbps data rate and a 10 MHz spread bandwidth yields aprocessing gain of 1000, or 30 dB. However, far greater processing gainsare achieved by impulse radio systems, where the same 10 kbps data rateis spread across a much greater 2 GHz spread bandwidth, resulting in atheoretical processing gain of 200,000, or 53 dB.

I. Capacity

It can be shown theoretically, using signal-to-noise arguments, that foran impulse radio system with an information rate of a few tens of kbps,thousands of simultaneous channels could be available as a result of itsexceptional processing gain.

The average output signal-to-noise ratio of a reference impulse radioreceiver may be calculated for randomly selected time-hopping codes as afunction of the number of active users, Nu, as:

${S_{out}\left( N_{u} \right)} = \frac{1}{\frac{1}{S_{out}(1)} + {\frac{1}{N_{s}}\frac{\sigma_{a}^{2}}{m_{p}^{2}}{\sum\limits_{k = 2}^{N_{u}}\left( \frac{A_{k}}{A_{1}} \right)}}}$

where Ns is the number of pulses integrated per bit of information, Alis the received amplitude of the desired transmitter, Ak is the receivedamplitude of interfering transmitter k's signal at the referencereceiver, and σ^(rec) ² is the variance of the receiver noise componentat the pulse train integrator output in the absence of an interferingtransmitter. The waveform-dependent parameters mp and σ_(a) ² are givenby

m_(p) = ∫_(−∞)^(∞)w(t)[w(t) − w(t − δ)] t andσ_(a)² = T_(f)⁻¹∫_(−∞)^(∞)[∫_(−∞)^(∞)w(t − s)υ(t) t]² s,

where w(t) is the transmitted waveform, υ(t)=w(t)−w(t−δ) is the templatesignal waveform, δ is the modulation time shift between a digital oneand a zero value data bit, Tf is the pulse repetition time, or frametime, and s is an integration parameter. The output signal to noiseratio that one might observe in the absence of interference is given by:

${S_{out}(1)} = \frac{\left( {A_{1}N_{s}m_{p}} \right)^{2}}{\sigma_{rec}^{2}}$

where, σ_(rec) ² is the variance of the receiver noise component at thepulse train integrator output in the absence of an interferingtransmitter. Further details of this analysis can be found in R. A.Scholtz, “Multiple Access with Time-Hopping Impulse Modulation,” Proc.MILCOM, Boston, Mass., Oct. 11-14, 1993.

J. Multipath and Propagation

One of the advantages of impulse radio is its resistance to multipathfading effects. Conventional narrow band systems are subject tomultipath through the Rayleigh fading process, where the signals frommany delayed reflections combine at the receiver antenna according totheir seemingly random relative phases resulting in possible summationor possible cancellation, depending on the specific propagation to agiven location. Multipath fading effects are most adverse where a directpath signal is weak relative to multipath signals, which represents asubstantial portion of the potential coverage area of a typical radiosystem. In a mobile system, received signal strength fluctuates due tothe changing mix of multipath signals that vary as the mobile unitsposition varies relative to fixed transmitters, other mobiletransmitters and signal-reflecting surfaces in the environment.

Impulse radios, however, can be substantially resistant to multipatheffects. Impulses arriving from delayed multipath reflections typicallyarrive outside of the correlation time and, thus, may be ignored. Thisprocess is described in detail with reference to FIGS. 5B and 5C. FIG.5B illustrates a typical multipath situation, such as in a building,where there are many reflectors 504B, 505B. In this figure, atransmitter 506B transmits a signal that propagates along three paths,the direct path 501B, path 1 502B, and path 2 503B, to a receiver 508B,where the multiple reflected signals are combined at the antenna. Thedirect path 501B, representing the straight-line distance between thetransmitter and receiver, is the shortest. Path 1 502B represents amultipath reflection with a distance very close to that of the directpath. Path 2 503B represents a multipath reflection with a much longerdistance. Also shown are elliptical (or, in space, ellipsoidal) tracesthat represent other possible locations for reflectors that wouldproduce paths having the same distance and thus the same time delay.

FIG. 5C illustrates the received composite pulse waveform resulting fromthe three propagation paths 501B, 502B, and 503B shown in FIG. 5B. Inthis figure, the direct path signal 501B is shown as the first pulsesignal received. The path 1 and path 2 signals 502B, 503B comprise theremaining multipath signals, or multipath response, as illustrated. Thedirect path signal is the reference signal and represents the shortestpropagation time. The path 1 signal is delayed slightly and overlaps andenhances the signal strength at this delay value. The path 2 signal isdelayed sufficiently that the waveform is completely separated from thedirect path signal. Note that the reflected waves are reversed inpolarity. If the correlator template signal is positioned such that itwill sample the direct path signal, the path 2 signal will not besampled and thus will produce no response. However, it can be seen thatthe path 1 signal has an effect on the reception of the direct pathsignal since a portion of it would also be sampled by the templatesignal. Generally, multipath signals delayed less than one quarter wave(one quarter wave is about 1.5 inches, or 3.5 cm at 2 GHz centerfrequency) may attenuate the direct path signal. This region isequivalent to the first Fresnel zone in narrow band systems. Impulseradio, however, has no further nulls in the higher Fresnel zones. Thisability to avoid the highly variable attenuation from multipath givesimpulse radio significant performance advantages.

FIGS. 5D, 5E, and 5F represent the received signal from a TM-UWBtransmitter in three different multipath environments. These figures areapproximations of typical signal plots. FIG. 5D illustrates the receivedsignal in a very low multipath environment. This may occur in a buildingwhere the receiver antenna is in the middle of a room and is arelatively short, distance, for example, one meter, from thetransmitter. This may also represent signals received from a largerdistance, such as 100 meters, in an open field where there are noobjects to produce reflections. In this situation, the predominant pulseis the first received pulse and the multipath reflections are too weakto be significant. FIG. 5E illustrates an intermediate multipathenvironment. This approximates the response from one room to the next ina building. The amplitude of the direct path signal is less than in FIG.5D and several reflected signals are of significant amplitude. FIG. 5Fapproximates the response in a severe multipath environment such aspropagation through many rooms, from corner to corner in a building,within a metal cargo hold of a ship, within a metal truck trailer, orwithin an intermodal shipping container. In this scenario, the main pathsignal is weaker than in FIG. 5E. In this situation, the direct pathsignal power is small relative to the total signal power from thereflections.

An impulse radio receiver can receive the signal and demodulate theinformation using either the direct path signal or any multipath signalpeak having sufficient signal-to-noise ratio. Thus, the impulse radioreceiver can select the strongest response from among the many arrivingsignals. In order for the multipath signals to cancel and produce a nullat a given location, dozens of reflections would have to be cancelledsimultaneously and precisely while blocking the direct path, which is ahighly unlikely scenario. This time separation of multipath signalstogether with time resolution and selection by the receiver permit atype of time diversity that virtually eliminates cancellation of thesignal. In a multiple correlator rake receiver, performance is furtherimproved by collecting the signal power from multiple signal peaks foradditional signal-to-noise performance.

In a narrow band system subject to a large number of multipathreflections within a symbol (bit) time, the received signal isessentially a sum of a large number of sine waves of random amplitudeand phase. In the idealized limit, the resulting envelope amplitude hasbeen shown to follow a Rayleigh probability density as follows:

${p(r)} = {\frac{r}{\sigma^{2}}{\exp\left( \frac{- r^{2}}{2\sigma^{2}} \right)}}$

where r is the envelope amplitude of the combined multipath signals, and2σ2 is the expected value of the envelope power of the combinedmultipath signals. The Rayleigh distribution curve in FIG. 5G shows that10% of the time, the signal is more than 10 dB attenuated. This suggeststhat a 10 dB fade margin is needed to provide 90% link reliability.Values of fade margin from 10 dB to 40 dB have been suggested forvarious narrow band systems, depending on the required reliability.Although multipath fading can be partially improved by such techniquesas antenna and frequency diversity, these techniques result inadditional complexity and cost.

In a high multipath environment such as inside homes, offices,warehouses, automobiles, trailers, shipping containers, or outside in anurban canyon or in other situations where the propagation is such thatthe received signal is primarily scattered energy, impulse radio systemscan avoid the Rayleigh fading mechanism that limits performance ofnarrow band systems, as illustrated in FIG. 5H and 5I. FIG. 5H depictsan impulse radio system in a high multipath environment 500H consistingof a transmitter 506H and a receiver 508H. A transmitted signal followsa direct path 501H and reflects off of reflectors 503H via multiplepaths 502H. FIG. 5I illustrates the combined signal received by thereceiver 508H over time with the vertical axis being signal strength involts and the horizontal axis representing time in nanoseconds. Thedirect path 501H results in the direct path signal 502I while themultiple paths 502H result in multipath signals 504I. UWB system canthus resolve the reflections into separate time intervals which can bereceived separately. Thus, the UWB system can select the strongest orotherwise most desirable reflection from among the numerous reflections.This yields a multipath diversity mechanism with numerous paths makingit highly resistant to Rayleigh fading. Whereas, in a narrow bandsystems, the reflections arrive within the minimum time resolution ofone bit or symbol time which results in a single vector summation of thedelayed signals with no inherent diversity.

K. Distance Measurement and Positioning

Impulse systems can measure distances to relatively fine resolutionbecause of the absence of ambiguous cycles in the received waveform.Narrow band systems, on the other hand, are limited to the modulationenvelope and cannot easily distinguish precisely which RF cycle isassociated with each data bit because the cycle-to-cycle amplitudedifferences are so small they are masked by link or system noise. Sincean impulse radio waveform has minimal multi-cycle ambiguity, it isfeasible to determine waveform position to less than a wavelength in thepresence of noise. This time position measurement can be used to measurepropagation delay to determine link distance to a high degree ofprecision. For example, 30 ps of time transfer resolution corresponds toapproximately centimeter distance resolution. See, for example, U.S.Pat. No. 6,133,876, issued Oct. 17, 2000, titled “System and Method forPosition Determination by Impulse Radio,” and U.S. Pat. No. 6,111,536,issued Aug. 29, 2000, titled “System and Method for Distance Measurementby Inphase and Quadrature Signals in a Radio System,” both of which areincorporated herein by reference.

In addition to the methods articulated above, impulse radio technologyin a Time Division Multiple Access (TDMA) radio system can achievegeo-positioning capabilities to high accuracy and fine resolution. Thisgeo-positioning method is described in U.S. Pat. No. 6,300,903, issuedOct. 9, 2001, titled “System and Method for Person or Object PositionLocation Utilizing Impulse Radio,” which is incorporated herein byreference.

J. Power Control

Power control systems comprise a first transceiver that transmits animpulse radio signal to a second transceiver. A power control update iscalculated according to a performance measurement of the signal receivedat the second transceiver. The transmitter power of either transceiver,depending on the particular setup, is adjusted according to the powercontrol update. Various performance measurements are employed tocalculate a power control update, including bit error rate,signal-to-noise ratio, and received signal strength, used alone or incombination. Interference is thereby reduced, which may improveperformance where multiple impulse radios are operating in closeproximity and their transmissions interfere with one another. Reducingthe transmitter power of each radio to a level that producessatisfactory reception increases the total number of radios that canoperate in an area without mutual interference. Reducing transmitterpower can also increase transceiver efficiency.

For greater elaboration of impulse radio power control, see U.S. Pat.No. 6,539,213, issued Mar. 25, 2003, titled “System and Method forImpulse Radio Power Control,” which is incorporated herein by reference.

II. Example Transceiver Implementation

A. Overview

FIG. 6 is an illustration of an example architecture for an impulseradio transceiver 600. Impulse radio 600 includes an antenna 602 coupledto an RF front-end 604. RF front-end 604 is coupled to a receiver RFsampling subsystem 606 for sampling RF receive signals and a transmitterpulser 608 for generating RF transmit impulses. Receiver RF samplingsubsystem 606 and pulser 608 are coupled to a timing subsystem 610 and acontrol subsystem 612. Timing subsystem 610 provides a sampling controlsignal 614 to receiver RF sampling subsystem 606, and a transmit timingcontrol signal 616 to pulser 608. Control subsystem 612 includes abaseband processor 620 and an impulse radio system controller 622 forcontrolling receive and transmit operations in impulse radio 600.Control subsystem 612 receives a timing signal 624 from timing subsystem610, and provides timing control commands 626 to the timing subsystem.

In transmit operation, baseband processor 620 provides a modulated datasignal 631 to pulser 608. In response to modulated data signal 631 andtransmit timing control signal 616 received from timing subsystem 610,pulser 608 generates an RF transmit impulse signal 632 and provides thesame to RF front-end 604. RF front-end 604 provides the transmit impulsesignal to antenna 602.

In receive operation, antenna 602 receives signals, for example, animpulse signal, and provides a received impulse signal to RF front-end604. RF front-end 604 in turn provides a conditioned, received impulsesignal 628 to receiver RF sampling subsystem 606. Receiver RF samplingsubsystem 606 samples conditioned, received impulse signal 628 inaccordance with sampling signal 614 received from timing subsystem 610,and provides a sampled impulse signal 630 to baseband processor 620 ofcontrol subsystem 612.

FIG. 7 is a more detailed block diagram of impulse radio transceiver600. RF front-end 604 includes a Transmit/Receive (T/R) switch 702coupled to antenna 602 and pulser 608 for isolating a transmit path froma receive path in impulse radio 600. T/R switch 702 provides a receivedsignal from antenna 602 to a Low Noise Amplifier (LNA)/RF filter 704.LNA/RF filter 704 provides an amplified and filtered received signal toan RF power-splitter 710 (also known as RF power divider 710) via avariable attenuator 706. RF power-splitter 710 divides the receivedsignal from variable attenuator 706 into a plurality of parallel RFpaths or channels. In one embodiment, RF splitter 710 divides thereceived signal four-ways to provide four RF receive channels 712 a, 712b, 712 c, and 712 d (collectively and generally referred to as receivechannels 712) to receiver RF sampling subsystem 606. The received RFsignal from variable attenuator 706 is present in each of the receivechannels 712.

B. RF Sampling Subsystem

Receiver RF sampling subsystem 606 includes four substantiallyidentical, parallel RF sampling channels 720 a, 720 b, 720 c, and 720 d(also referred to as “RF samplers” or just “samplers” 720 a-720 d). Eachof receive channels 712 a-712 d output from power-splitter 710 isprovided to a respective one of parallel RF samplers 720 a-720 d. Sinceeach RF sampler is substantially identical to each of the other RFsamplers, the following description of RF sampler 720 a suffices for theother RF samplers. RF sampler 720 a includes an input amplifier 722 afor amplifying an RF received signal received from associated receivechannel 712 a. Amplifier 722 a provides an amplified RF received signal724 a to a pair of RF sampling correlators, including a first samplingcorrelator 726 a and a second sampling correlator 727 a associated withthe first sampling correlator. First sampling correlator 726 acorrelates RF received signal 724 a with sampling pulses derived from asampling control signal (736 a, discussed below), and provides aresulting first Sample/Hold (S/H) signal 728 a, representing correlationresults, to baseband processor 620.

Similarly, second sampling correlator 727 a correlates RF receivedsignal 724 a with sampling pulses time synchronized with but slightlytime offset from the sampling pulses derived from the sampling controlsignal (736 a) provided to associated correlator 726 a, and provides aresulting second Sample/Hold (S/H) signal 729 a, representingcorrelation results, to baseband processor 620. Thus, samplingcorrelators 726 a and 727 a respectively produce first and secondreceived signal samples slightly offset in time from one another.

Similarly, the other RF samplers 720 b, 720 c, and 720 d respectivelyprovide S/H baseband signal pairs (728 b, 729 b), (728 c, 729 c), and(728 d, 729 d) to baseband processor 620. Correlators 726 a-726 d, andrespectively associated correlators 727 a-727 d operate as a pluralityof single-stage down-converters for directly down-converting thereceived RF signal (in RF channels 712) to sampled baseband. Therefore,S/H signals 728 a-728 d and S/H signals 729 a-729 d are also referred toas received, sampled baseband signals 728 a-728 d and 729 a-729 d. Forconvenience, correlators 726 a-726 d and 727 a-727 d are alsocollectively and generally referred to as correlators 726 and 727,respectively. Also, S/H signals 728 a-728 d and 729 a-729 d arecollectively and generally referred to as S/H signals 728 and 729,respectively.

C. Timing Subsystem

Timing subsystem 610 includes a master oscillator 732 and a plurality,such as four, Precision Timing Generators (PTGs) (also referred to asadjustable timers) 734 a, 734 b, 734 c, and 734 d, each associated witha respective one of RF samplers 720 a, 720 b, 720 c, and 720 d. Forconvenience, adjustable timers 734 a-734 d are collectively andgenerally referred to as adjustable timers 734. Master oscillator 732provides a common reference clock signal to receiver RF samplingsubsystem 606, timing subsystem 610, and controller subsystem 612.

Adjustable timer 734 a receives a timing control signal 735 a (alsoreferred to as a timing control command 735 a) from baseband processor620, and derives sampling control signal 736 a (mentioned above) basedon the timing control command. Adjustable timer 734 a provides samplingcontrol signal 736 a to RF sampler 720 a to control when RF sampler 720a samples the received signal, as described above. Adjustable timers 734b-734 d (collectively and generally referred to as adjustable timers734) are arranged and operate in a similar manner with respect toassociated RF samplers 720 b-720 d and baseband processor 620. Inaddition, baseband controller 620 can control each of adjustable timers734 independently. In this manner, baseband processor 620 controls whenRF samplers 720 sample the received signal in receiver 600.

In the depicted embodiment, a fifth adjustable timer 740 (also referredto as transmit timer 740) receives a transmit timing control signal 735e (also referred to as a transmit timing control command 735 e) frombaseband processor 620, and derives a transmit trigger signal 741 basedon the transmit timing control command. Transmit time 740 providestransmit timing control signal 741 to transmitter pulser 608 to controlwhen the pulser generates a transmit impulse. In another embodiment, thetransmit trigger signal (for example, signal 741) can be provided by oneof the PTGs (for example, PTG 734 d), whereby transmit timer 740 can beeliminated to reduce a radio part count.

PTGs 734 a-734 d can be controlled (in a manner described below) suchthat respective sampling control signals 736 a-736 d can be timesynchronized and coincident with each other, time synchronized butoffset with respect to each other, or asynchronous with respect to eachother. Correspondingly, PTGs 734 a-734 d can trigger respectivecorrelators 726 a-726 d (and associated correlators 727 a-727 d) torespectively sample receive channels 712 a-712 d synchronously andcoincidentally, synchronously but offset in time with respect to oneanother, or asynchronously with respect to each other. Correlators (suchas correlators 726 a-726 d) and adjustable timers (such as timers 734a-734 d) associated with the correlators can be added or removed asnecessary to meet the requirements of any particular impulse radio basedreceive and/or transmit application. Also, PTG 740 (the transmit timer)can be controlled such that transmit trigger signal 741 can be timesynchronized and coincident with one or more of sampling control signals736 a-736 d, time synchronized but offset with respect to the samplingcontrol signals, or asynchronous with respect to the sampling controlsignals.

D. Control Subsystem

Control subsystem 612 includes baseband processor 620 for implementingvarious transmit and receive signal processing functions, and forperforming various receive and transmit control functions in impulseradio 600, as described above, and as will be further described below.Control subsystem 612 also includes system controller or processor 622coupled to a memory 766 and a user interface 768. Baseband processor620, system controller 622, memory 766 and user interface 768 arecoupled together, and intercommunicate with one another, over aprocessor bus 770 including an address bus and a data bus. A buscontroller 771 coupled to processor bus 770 assists in controllingtransfers of data, information, and commands between the abovementionedelements coupled to the processor bus. For example, bus controller 771arbitrates between various users of processor bus 770 based on datatransfer priorities, and the like.

System controller 622 provides high level control over impulse radio600. System controller 622 can receive inputs, such as user commands anddata, via an input/output device (not shown) connected to user interface768. Also, system controller 622 can send data to the input/outputdevice via user interface 768. System controller 622 can send commandsand data to baseband processor 620, and can receive data from thebaseband processor. Information received through user interface 768 canbe provided to memory 766.

E. Baseband Processor

Over processor bus 770, baseband processor 620 can request and receiveinformation and commands, used for the baseband signal processing andcontrol functions, from both memory 766 and system controller 622.Baseband processor 620 provides dedicated timing control commands 735a-735 d (collectively and generally referred to as timing controlcommands 735) to each of PTGs 734 to respectively control the timing ofsampling control signals 736, as described above. In this manner,baseband processor 620 can independently control when each of RFsamplers 720 samples the received signal. In an alternative embodiment,baseband processor 620 can provide the timing control commands to PTGs736 over an extended processor bus, similar to processor bus 770,coupled between baseband processor 620 and timing subsystem 610. Inaddition, baseband processor 620 provides demodulated data to andreceives information (for example, to be modulated) from a datasource/sink 780.

Baseband processor 620 includes a plurality of Analog-to-Digitalconverters (A/Ds) to digitize baseband signals 728 and 729 received fromreceiver RF sampling subsystem 606. For example, a pair of such A/Dsassociated with RF sampler 720 a includes first and second A/Ds 772 aand 773 a to respectively digitize S/H baseband signals 728 a and 729 a,to produce respective digitized baseband signals 774 a and 775 a. A/Ds772 a and 773 a provide respective digital baseband signals 774 a and775 a to a digital baseband signal bus 777 coupled to the various signalprocessing functions of baseband processor 620. Further basebandprocessor A/D pairs (772 b, 773 b), (772 c, 773 c) and (772 d, 773 d)are arranged and operate in a similar manner with respect to associatedRF samplers 720 b-720 d and digital baseband signal bus 777. Forconvenience, A/Ds 772 a-772 d and 773 a-773 d are collectively andgenerally referred to as A/Ds 772 and 773, respectively. Similarly,digital baseband signals 774 a-774 d and 775 a-775 d are collectivelyand generally referred to as digital baseband signals 774 and 775,respectively.

Digital baseband signals 774 and 775 can include trains of digital datasamples. Therefore, baseband processor 620 includes a data memory, suchas a register buffer, Random Access Memory, or the like, to store thedigital data samples, whereby the digital data samples are available tothe baseband signal processing and control functions of the basebandprocessor.

Baseband processor 620 includes a plurality of signal processingfunctional blocks, such as, but not limited to:

-   -   1) radio controller 779;    -   2) a timer control 781;    -   3) a signal acquirer 782, including a signal detector 782 a and        a signal verifier 782 b;    -   4) a data modulator 784 and a data demodulator 786;    -   5) a received signal tracker 788;    -   6) a link monitor 790; and    -   7) an interference canceler controller 792.

The various signal processing functional blocks mentioned above canexchange information/signals with one another, as necessary, using knowntechniques. For example, such an exchange of information/signals canoccur over a signal processor communication bus 794, coupled between thesignal processing functional blocks, within baseband processor 620.

Radio controller 779 performs various control functions within basebandprocessor 620. Radio controller 779 can receive data from and pass datato processor bus 770 and data source/sink 780. Radio controller 779performs low level protocol handling. For example, radio controller 779can function as an intermediate protocol handler between modulator 784(or demodulator 786) and either of system controller 622 and datasource/sink 780. For example, radio controller 779 can receive datapackets from system controller 622, and then partition the data packets,encode the partitioned data packets, and dispatch the partitioned,encoded data packets to the modulator. Radio controller 779 can alsocalibrate A/Ds 772 and 773, and control variable attenuator 706 in RFfront end 604.

Data modulator 784 modulates information data received from datasource/sink 780, and communicates modulated data to pulser 608 forsubsequent RF transmission from antenna 602. In one embodiment, datamodulator 784 derives transmit timing control command 735 e based on themodulated data. In response to transmit timing control command 735 e,transmit timer 740 derives transmit trigger 741. In this manner, datamodulator 784 controls triggering of pulser 608 in accordance with themodulated data derived by the data modulator.

Data demodulator 786 demodulates digitized baseband signals 774 and 775produced by respective A/Ds 772 and 773 to recover informationtransmitted, for example, from a remote impulse radio transmitter. Forexample, data demodulator 786 demodulates received symbols in basebandsignals 774 and 775. The recovered information can be provided to datasource/sink 780. Data demodulator 786 can implement all of the signalprocessing functions necessary to support any given application. Forexample, data demodulator 786 can include an impulse amplitudeaccumulator for accumulating impulse amplitudes, logic to effectdemodulation decisions, logic to measure an impulse amplitude and areceived impulse Time-of-Arrival (TOA), and so on, as needed to supportany now known or future communication and/or radar applications, as wellas to determine a separation distance between impulse radios based onamplitude, and so on. Data demodulator 786 also provides information tothe other signal processing functions of baseband processor 620.

Signal Tracker 788 locks onto and tracks the timing of a receivedimpulse signal represented by digitized baseband signals 774 and 775produced by A/Ds 772 and 773. In one embodiment, signal tracker 788cooperates with an RF sampler (for example, RF sampler 720 a), anadjustable timer associated with the RF sampler (for example, timer 734a and/or oscillator 732), and timer control 781, to form a Lock Loop forderiving a system timing signal (such as a sampling control signal),indicative of impulse TOAs in the received impulse signal, and used tosample impulses in the impulse signal. The system timing signal derivedby the above mentioned Lock Loop can be made available to all of thesignal processing functional blocks in baseband processor 620. Based onthis system timing signal, baseband processor 620 can provide timingcontrol commands to each of PTGs 734 to control when each of theassociated correlators 726 and 727 samples the received signal, inrelation to, for example, a received impulse signal.

Timer control 781 receives timing information from the other signalprocessing functional blocks in baseband processor 620 and translatesthe timing information into timing control commands compatible with PTGs734. Timer control 781 also manages the delivery of the timing controlcommands to the PTGs 734. Timer control can also include Lock Loopelements, such as a PN code generator, and the like, to assist signaltracker 788 in deriving system timing.

Link Monitor 790 monitors a received impulse signal, as represented bydigitized baseband signals from A/Ds 772 and 773, and demodulatedinformation provided by demodulator 786, to determine, inter alia,transmitter-receiver propagation link performance and impulse signalpropagation characteristics. Link monitor 790 determines such linkperformance and propagation characteristics based on received signalquality measurements, such as received impulse signal-to-noise level,symbol error rate, and so on. Based on such determined link performance,link monitor 790 provides an attenuator control command 796 to variableattenuator 706 in RF front-end 604, thereby commanding the variableattenuator to a desirable attenuation setting.

Interference canceler controller 792 implements interference canceleralgorithms and controls interference canceling in impulse radio 600, toeffect interference canceling in accordance with the various embodimentsdisclosed in the applications entitled “Method and System for ReducingPotential Interference in an Impulse Radio” and “Method and System forCanceling Interference in an Impulse Radio”, which have beenincorporated by reference above.

F. Paired Correlators

The paired correlators in each of RF samplers 720 can be arranged tosample a received signal in such a way as to support, inter alia,various types of modulation and demodulation techniques, such as thosedescribed in U.S. patent application Ser. No. 09/538,519, filed Mar. 29,2000, entitled “Vector Modulation System and Method for Wideband ImpulseRadio Communications,” and U.S. patent application Ser. No. 09/537,692,filed Mar. 29, 2000, entitled “Apparatus, System and Method for FlipModulation in an Impulse Radio Communication System.” Accordingly, thefirst and second correlators in each RF sampler are respectivelytriggered to sample the received signal at first and second samplingtimes that are synchronized and slightly time offset from one another,as is now more fully described.

FIG. 8A is an illustration of impulse 102 transmitted by a remoteimpulse radio and received by antenna 602. Impulse 102 passes through aseries of receiver components (such as RF front end 604, amplifier 722a, and so on, as described above) in a receive path of impulse radio 700before the signal arrives at an input to any one of sampling correlators726 and 727. Such a receive path, leading into any one of correlators726 and 727, has a receive response (that is, a time-domain receive pathresponse) to applied impulse 102. The receive path response is based onthe individual responses of each of the receive path components to theimpulse 102. FIG. 8B is an illustration of an example receive pathresponse 804. Receive path response 804 has a cycle period TIRapproximately equal to, but not necessarily the same as, a cycle periodof transmitted impulse 102.

To take advantage of the above mentioned modulation and demodulationtechniques, such as vector modulation and demodulation, the first andsecond correlators (for example, correlators 726 a and 727 a) in eachpair of correlators in impulse radio 700 can be arranged to sample thereceived signal in the following manner: the first correlator samplesthe received signal at a first sample time tS1 to produce a firstreceived signal sample 812 (for example, as depicted in FIG. 8B); andthe second correlator samples the received signal at a second sampletime tS2, spaced in time from the first sample time tS1 by a timeinterval that is a fraction of receive path response cycle period TIR,to produce a second (delayed) received signal sample 814. In oneembodiment, first sample 812 and second sample 814 are spaced in timefrom one another by a time interval TIR/4 (that is, by a quarter ofreceive path response cycle period TIR). When first and second samples812 and 814 are spaced from each other by a quarter of a cycle ofreceive path response 804, first and second samples 812 and 814 are“in-quadrature” (that is, the first and second samples have a quadraturerelationship to one another, with respect to receive path response 804),and thus can be referred to as an In-phase (I) and Quadrature (Q) samplepair (also referred to as a sample pair), where first sample 812 is theI sample, and delayed sample 814 is the Q sample.

In other embodiments, and more generally, second sample 814 can bedelayed from first sample 812 by a time delay different from a quarterof a cycle of receive path response 804, whereby the first and secondsamples are no longer in-quadrature. Since first sample 812 and second,delayed sample 814 can be separated by other than a quarter of a cycleof receive path response 804, first sample 812 and second sample 814 aremore generally referred to as a reference “I” sample and a delayed “J”sample, respectively. This generalized first I sample and second Jsample (I-J sample pair) naming convention is introduced and furtherdescribed in U.S. patent application Ser. No. 09/538,519, filed Mar. 29,2000, entitled “Vector Modulation System and Method for Wideband ImpulseRadio Communications,” mentioned and incorporated by reference above.The generalized I-J sample pair naming convention is used in thedescription below, with the understanding that the delayed J sample (forexample, sample 814) can be delayed relative to the reference I sample(for example, sample 812) by a time delay less than, equal to, or morethan a quarter of a cycle of receive path response 804. Moreover, it isto be understood the time delay between the I and J samples can becontrolled in a receiver of the present invention to support properoperation of the receiver in any impulse radio application requiring thetime delay, such as vector demodulation, for example. A mechanism bywhich the time delay can be controlled is not the subject of the presentinvention, and therefore, is discussed no further.

FIG. 9 is a block diagram of an example correlator pair arrangement 900,corresponding to RF sampler 720 a, for example. Correlator pairarrangement 900 includes a first correlator 902 (I correlator) and asecond correlator 904 (J correlator) (respectively corresponding tofirst and second correlators 726 a and 727 a, for example). Adjustabletimer 734 a provides sampling control signal 736 a to a sampling pulsegenerator 906.

In response to sampling control signal 736 a, sampling pulse generator(also referred to as a pulse shaping circuit) 906 derives a firstsampling signal 908 having an amplitude characteristic (that is, pulseshape) determined by the sampling pulse generator. Pulse shaping circuit906 provides first sampling signal 908 to first correlator 902 and to adelay 920. First correlator 902 preferably comprises a multiplierfollowed by a short term integrator to sum the multiplied productbetween received signal 724 a and first sampling signal 908. Firstcorrelator 902 preferably includes a sample-and-hold circuit at anoutput of the integrator for storing a correlation result, so as toproduce S/H signal 728 a. In this manner, first correlator 902 samplesreceived signal 724 a in accordance with first sampling signal 902 toproduce S/H signal 728 a (which includes I samples).

Delay 920 delays first sampling signal 908 by a fraction of cycle periodTIR (such as quarter cycle period TIR/4) as described above, to producea delayed sampling signal 922 (also referred to as a second samplingsignal 922). Delay 920 provides delayed sampling signal 922 to secondcorrelator 904. Second correlator 904 samples received signal 724 a inaccordance with delayed sampling signal 922 to produce S/H signal 729 a(which includes J samples).

In an alternative embodiment, sampling pulse generator 906 isincorporated into adjustable timer 734 a, whereby adjustable timer 734 aprovides a sampling signal directly to both correlator 902 and delay920. In another embodiment, either or both of sampling pulse generator906 and delay 920 can be incorporated into correlator 902, wherebyadjustable timer 734 a provides sampling control signal 736 a directlyto correlator 902.

FIG. 10A is an example timing waveform representing sampling controlsignal 736 a. Sampling control signal 736 a includes a train of pulses1002.

FIG. 10B is an example timing waveform representing first samplingsignal 908, derived by sampling pulse generator 906. First samplingsignal 908 includes a train of sampling pulses 1004, each correspondingto an associated one of pulses 1002. Each of the sampling pulses 1004 isapproximately square shaped for practical reasons, however, samplingpulse generator 906 can derive sampling pulses having other shapes. Forexample, each of the sampling pulses can have a pulse shapesubstantially equivalent to received impulses in a received impulsesignal. For example, if the impulse radio antenna differentiatestransmitted impulses (received at the antenna), then sampling signal 908can consist of pulses that are substantially equivalent to the firstderivative of the transmitted impulses. From a practical standpoint,sampling signal 908 consists of square pulses since square pulses can begenerated with less complex receiver logic.

Each of sampling pulses 1004 directly controls receive signal samplingby correlator 902. That is, correlator 902 correlates received signal724 a with each of sampling pulses 1004 during a time intervalcorresponding to a width 1006 (also referred to as a sampling window1006) of the sampling pulses 1004. The width of each of sampling pulses1004 is preferably less than ½ the pulse width of a received impulse andcentered about a center amplitude peak of the received impulse. Forexample, where received impulses are approximately 0.5 ns wide, thesquare pulses are preferably approximately 0.125 ns wide.

FIG. 10C is an example timing waveform representing second samplingsignal 922, produced by delay 920. Second sampling signal 922 includes atrain of sampling pulses 1008, each delayed with respect to anassociated one of pulses 1004. Pulses 1008 control receive signalsampling by correlator 904 in the same manner pulses 1004 controlreceive signal sampling by correlator 902.

Impulse radio 600, described above in detail in connection with FIGS. 6and 7, and the further impulse radio functionality described above indetail in connection with FIGS. 8-9, and 10A-10C, together represent aninterrelated collection of impulse radio functional blocks (orfunctional building blocks) from which different impulse radioembodiments (including, for example, receiver architectures and methods)can be constructed, in accordance with the principles of presentinvention. Accordingly, the impulse radio receivers (or receiversubsystems) described below, which operate in accordance with theexample methods of the present invention, also described below, includemany of the impulse radio functional blocks described above.

For convenience, any impulse radio functional block and/or signaloriginally described above (for example in connection with FIG. 7 andFIG. 9), shall retain its original reference designator (as designated,for example, in FIG. 7 and FIG. 9) when it is included in a subsequentimpulse radio embodiment, such as those described below. The originalreference designator shall be retained even when the function orcharacteristics of the originally described functional block and/orsignal is slightly modified by or slightly different in the subsequentembodiment. However, any difference between the original and subsequentfunctionality shall be described.

G. Lock Loop

A description of an exemplary lock loop used for tracking an impulsesignal (e.g., 1102) is now explained with reference to FIG. 11, whichshows a portion of impulse radio transceiver 600. In the presentinvention, such tracking occurs after frame first stage acquisition ofthe present invention, where course frame alignment is achieved and codesynchronization is achieved to the extent of an acquisition short codeof the present invention. First stage acquisition of the presentinvention is described in more detail below.

While transceiver 600 is tracking, correlator 726 a (also referred to assampler 726 a) samples received signal 724 a at sample times coincidingwith expected impulses in received signal 724 a. More specifically,precision timing generator (PTG) 734 a receives a periodic timing signal1156 from a time base 1150 (e.g., oscillator 732 in FIG. 7). Time base1150 is adjustable and controllable in time, frequency, or phase, asrequired by the lock loop in order to lock on received signal 724 a. Acode source 1152 receives a timing signal 1154 from time base 1150(and/or a synchronization signal 1158 from PTG 734 a). PTG 734 a alsoreceives a code control signal 735 a (also referred to as a timingcontrol command 735 a) from code source 1152 of baseband processor 620.PTG 734 a utilizes the periodic timing signal 1156 and code controlsignal 735 a to produce coded timing signal 736 a. Sampling correlator726 a samples received signal 724 a, based on coded timing signal 736 a,and outputs S/H baseband signal 728 a. A/D 772 a converts basebandsignal 728 a to a digitized baseband signal 774 a. Digitized basebandsignal 774 a can be, for example, used for further stages of signalacquisition or demodulation, depending on the situation. Furtherexamples and details of correlation and sampling processes can be foundin the above-referenced patents and U.S. Pat. No. 6,421,389, issued Jul.16, 2002, entitled “Baseband Signal Converter Device for a WidebandImpulse Radio Receiver,” which is incorporated herein in its entirety byreference.

Digitized baseband signal 774 a is also provided to a lock loop filter1148 (e.g., a low pass filter). The output of filter 1148 is a filterederror signal 1168, which provides adjustments to adjustable time base1150 to time position the periodic timing signal 1156 in relation toreceived signal 724 a. In other words, a lock loop including filter1148, time base 1150, PTG 734 a, correlator 726 a, and A/D 772 a is usedto generate a filtered error signal 1168, which adjusts time base 1150.

As explained throughout this application, multiple samples are oftenaccumulated (i.e., integrated) for integration gain. For example, asshown in FIG. 11, an accumulator 1114 (which can be a ramp builder ofthe present invention) accumulates a plurality of digitized basebandsignal values of digitized baseband signal 774 a to produce anaccumulated digitized baseband signal 1116. Accordingly, Lock Loopfilter 1148 can derive timing error signal 1168 based on accumulatedsignal 1116, rather than signal 774 a. This accumulated signal 1116preferably consists of ramp values produced by a ramp builder of thepresent invention.

The above description is just one example of a lock loop that can beused to track a received impulse signal. Those of ordinary skill in therelevant art will appreciate that tracking can be accomplished invarious other manners while still being within the spirit and scope ofthe present invention.

III. Preferred Embodiments of the Present Invention

A. Terminology

To more clearly explain the present invention, an effort is madethroughout the specification to adhere to the following term definitionsas consistently as possible.

The term “packet” refers to a plurality of frames, including headerframes followed by data payload frames. One or more trailer frames canfollow the data payload frames. A packet can include a delimiter thatseparates the header and the data payload. A packet can also include oneor more ratchet codes of the present invention. A packet can alsoinclude command bits prior to the data payload. Packets are discussed inmore detail with reference to FIGS. 12A and 12B.

The term “frame” refers to a fixed interval of time (e.g., 100 nsec)within which one or more impulses can be positioned. Each frame has astart time and an end time, each defining a frame boundary. A frame ofan impulse signal received by an impulse radio receiver is oftenreferred to as a “received frame”.

The term “sample frame” refers to a fixed interval of time within whicha receiver attempts to sample one or more impulses of a received signal.Each sample frame has a initial frame time and an end time, eachdefining a sample frame boundary. A received frame and a sample frameare not the same entity. A sample frame is a receivers guess as to whena received frame occurs.

The term “frame offset” refers to the time separating a boundary (i.e.,the start or end boundary) of a received frame and one of the twoclosest boundaries of a sample frame. If a boundary of a sample frame isroughly aligned (in time) with a boundary of a received frame, there isno frame offset (i.e., FO≈0). When a boundary of a sample frame is notaligned with a boundary of a received frame, there is a frame offsetdefined by the time separating the boundaries (i.e., FO>0; e.g., FO=32nsec).

The term “frame alignment” refers to the extent to which sample frames(i.e., of receiving equipment) are time aliged (i.e., synchronized) withrespect to receive frames of a received signal. When the frames areproperly aligned (i.e., frame alignment has been achieved) there is noframe offset (i.e., FO≈0), and it can be said that the frames have beensynchronized.

The term “code” refers to a rule defining a time ordered series offrames and one or more positions of impulses within each of the frames.The position of an impulse within a frame (also referred to as animpulse position) refers to the time offset between the start of a frameand the start of the impulse.

The term “short acquisition code” refers to a relatively short code (ascompared to a code used to encode data) that is defined by a packetprotocol. The short acquisition code defines where impulses arepositioned within specific frames of a packet header. A shortacquisition code is also referred to as a “repeating short acquisitioncode”.

The term “acquisition code sequence” refers to a time ordered series offrames that are defined by the short acquisition code. Morespecifically, the short acquisition code defines where one or moreimpulses are positioned within each frame of the time ordered series offrames. An acquisition code sequence is also referred to as a “repeatingacquisition code sequence” because a plurality of identical acquisitioncode sequences are included in a packet header. An acquisition codesequence is used by an impulse radio receiver to acquire both properframe alignment and short code synchronization in accordance withembodiments of the present invention.

The term “long code” refers to a relatively long code (as compared tothe short code) that it used to encode information (i.e., data) in adata payload portion of a packet.

The term “code offset” refers to where in a code sequence a receivedsignal is as compared to a given time. For example, a code offset canrefer to where in a received signal a code sequence actually is ascompared to where a receiver thinks the code is. This will be furtherexplained in connection with FIGS. 13, 18, and 19.

The term “delimiter” refers to one or more frames in a packet that isused to indicate the end of the header and the beginning of the datapayload in the packet.

The term “ratchet code” refers to one or more frames in a packet header,following the last repeating acquisition code sequence, that enables areceiver to resolve the ambiguity that results from an integrationlength (see below) being greater than the number of ramp builders. Theone or more ratchet codes in a packet is/are located prior to thedelimeter (i.e., between the last acquisition code sequence and thedelimiter).

The term “integration length” refers to the number of samples of areceived signal that are accumulated (e.g., by an integrator or rampbuilder) prior to the receiver making a decision. Typically, a greaterintegration length provides increased integration gain, and thus, animproved signal to noise (S/N) ratio. The integration length is alsoreferred to as the “ramp integral length,” which is often designated“N”.

The term “ramp builder” refers to a portion of hardware and/or receiverfunctionality that is used to accumulate (i.e., integrate) samples of areceived signal.

The term “accumulated ramp value” refers to a sum of a plurality ofsamples of a received signal. Within this specification, the term“accumulated ramp value” typically refers to the output of a rampbuilder. If a receiver includes a plurality of ramp builders, then thereceiver can produce a plurality of accumulated ramp values in parallel.For example, if a receiver includes four (4) ramp builders, the receivercan produce four (4) accumulated ramp values in parallel (that is,concurrently).

The term “protocol” refers to a specific set of rules, procedures orconventions relating to format and timing of transmissions between twodevices.

The term “packet protocol” refers to the format of an impulse signalpacket, which is known by impulse radios in communications with oneanother. A packet protocol according to an embodiment of the presentinvention defines: the integration length (also referred to as the rampinterval length N); the length of a repeating short acquisition code;the actual short acquisition code sequence; the number of shortacquisition code sequences per packet; the length of each ratchet codesequence; each actual ratchet code sequence; the number of differentunique ratchet code sequences per packet; the number of each differentunique ratchet code sequence per packet; the actual delimeter sequence;the long code used to encode the data payload; and the length of thedata payload.

The term “receiver sample timeline” (also referred to as sampletimeline) refers to a timeline established by a receiver to define whenthe receiver samples a received signal. The timeline has a start timethat indicates when the receiver begins to sample the received signal.The receiver sample timeline includes a series of sample frames, whereinthe start time of the receiver sample timeline identifies the startboundary of an initial sample frame (of the series of sample frames).

B. Exemplary Packets

It is useful to describe an exemplary packet of a received impulsesignal prior to explaining the present invention in detail.

FIG. 12A illustrates an exemplary packet 1202 transmitted by an impulseradio transmitter. Packet 1202 includes a header portion 1203 (alsoreferred to as a header 1203) followed by a data payload portion 1208(also referred to as a data payload 1208). Header 1203 includes aplurality of acquisition short code sequences 1204 (referred to as aplurality of short codes 1204) followed by a delimiter 1206. Theplurality of acquisition short code sequences 1204 are referred to as arepeating short code when each of the plurality of short codes areidentical to one another, as depicted in FIG. 12A. Such repeating shortcode sequences are used by an impulse radio receiver to quickly andefficiently acquire both frame and code synchronization, in a mannerdescribed below. Delimiter 1206 indicates an end of header 1203 and abeginning of data payload 1208.

Exemplary details of an acquisition code sequence 1204 are depicted asan exploded view 1210 in FIG. 12A. As depicted in exploded view 1210,the exemplary acquisition code sequence includes four consecutive framesFrame 0 (F0), Frame 1 (F1), Frame 2 (F2) and Frame 3 (F3), andtherefore, has a length of four frames. The exemplary short acquisitioncode sequence repeats every four frames, and thus is referred to as amodulo-4 (i.e., length 4) short acquisition code sequence. In thisexample, each frame (e.g., F0, F1, F2, or F3) includes an impulse thatis located at a predefined one of four different positions as defined bythe code, where P0 represents a zeroth position, P1 represents a firstposition, P2 represents a second position, and P3 represents a thirdposition. In the example shown, the exemplary acquisition code is P0,P1, P2, P3. That is, the exemplary acquisition code sequence consists ofan impulse in position P0 in zeroth frame F0, an impulse in position P1in first frame F1, an impulse in position P2 in second frame F2, and animpulse in position P3 in third frame F3. Stated another way, the hopsequence of the acquisition short codes is P0, P1, P2, P3. The length ofthe short acquisition code is preferably equal to the number of rampbuilders (in the receiver) that are used by the receiver to acquire animpulse signal so that the ramp builders may operate concurrently toquickly and efficiently achieve initial frame alignment. Thus, in theabove example, it is assumed that the receiver includes four rampbuilders (a portion of hardware and/or receiver functionality, discussedin more detail below) that can be used to acquire a received impulsesignal.

Exemplary details of frame F0 are depicted in exploded view 1212 of FIG.12A, wherein the width of frame F0 (and of each of the other framesF1-F3) is 100 nsec, and the width of each impulse within the frame isapproximately 0.5 nsec. As shown in view 1212, the impulse is inposition P0 in zeroth frame F0. Impulses at positions P1, P2 and P3 areshown in dotted line to illustrate alternative pulse positionpossibilities that impulses could occupy in accordance with alternativecodes of the present invention. It is noted that exploded view 1212, andthe rest of FIG. 12A, are not necessarily drawn to scale.

The acquisition code sequence P0, P1, P2, P3 is just one example of amodule-1 acquisition code. Another example of a modulo-4 acquisitioncode is P1, P3, P0, P2. Two further examples of modulo-4 acquisitioncodes include: 1) P0, P2, P3, P1; and 2) P0, P1, P3, P2. As will beexplained in more detail, alternative code lengths can be used (e.g., amodulo-8 acquisition code, a modulo-16 acquisition code, etc.).

Following the plurality of repeating acquisition code sequences 1204 isa delimiter 1206. Delimiter 1206 separates header 1203 from data payload1208 and indicates that the frames following the delimiter are frames ofdata payload 1208 (e.g., that include symbols to be demodulated). In oneembodiment, delimiter 1206 includes the same number of frames as shortacquisition code sequence 1204. Thus, according to this embodiment andthe example shown in FIG. 12A, delimiter 1206 is four (4) frames inlength. In another embodiment, described below in reference to FIG. 12B,delimiter 1206 includes an integer multiple of the number of framesincluded in the acquisition code sequence 1204. Following delimiter 1206is data payload 1208, which, for example, includes information to bedemodulated by an impulse radio receiver.

According to an embodiment of the present invention, the delimiter isidentical to the acquisition code sequence except that each impulse isflipped. Thus, based on the example shown in FIG. 12A, the delimiterwould include four frames, with a flipped impulse in position P0 in theframe F0, a flipped impulse in position P1 in frame F1, a flippedimpulse in position P2 in frame F2, and a flipped impulse in position P3in frame F3. This is illustrated in FIG. 14, which shows an exemplaryacquisition code sequence 1204 and its corresponding delimiter 1206.Systems and methods for producing flipped impulses are disclosed incommonly assigned co-pending U.S. patent application Ser. No.09/537,692, filed Mar. 29, 2000, entitled, “Apparatus, System and Methodfor Flip Modulation in an Impulse Radio Communication System,” which isincorporated herein by reference in its entirety.

Returning to the discussion of FIG. 12A, according to one exemplaryembodiment, packet 1202 includes forty (40) short acquisition codesequences 1204, followed by one (1) delimiter 1206, followed by onethousand (1000) bits worth of frames (e.g., 1000*N frames, where N isthe data integration length) of payload data 1208. One of ordinary skillin the art will appreciate that packets including other numbers of shortcode sequences, delimiters, and frames of data payload are within thespirit and scope of the present invention.

Referring now to FIG. 12B, according to another embodiment of thepresent invention, header 1203 (of packet 1202) includes one or more“ratchet” codes 1205, prior to delimeter 1206. Ratchet codes 1205, whichare used when samples are integrated to improve a signal to noise ratio,enables a receiver to resolve the ambiguity resulting from theintegration length being greater than the number of ramp builders, aswill be described later. The length of a ratchet code 1205 is typicallyan integer multiple of the length of short acquisition code 1204. Whenpacket 1202 includes one ratchet code 1205, the length of delimiter 1206is the same as the length of the ratchet code. If more than one ratchetcode 1205 is included in packet 1203, and at least two of the ratchetcodes 1205 are of different length, then the length of the delimiter isthe same as the length of the longest ratchet code 1205. The abovementioned ambiguity resulting from the integration length being greaterthan the number of ramp builders, along with additional details ofratchet codes 1205 and delimeter 1206, are discussed in more detailbelow.

C. Problem Description

When an impulse radio receiver begins receiving a packet (such as packet1202 described above), the impulse radio receiver initially establishesa receiver sample timeline based on an apriori knowledge of the framewidth and code associated with the received impulse signal. The impulseradio receiver uses the receiver sample timeline to sample the receivedpacket of the receive impulse signal. Initially, the receiver sampletimeline established by the impulse radio receiver is most likelyneither frame synchronized nor code synchronized with the receivedimpulse signal. This initial situation is illustrated by way of examplewith reference to FIG. 13.

FIG. 13 is an illustration of an example timing relationship between aportion of a received impulse signal 1301 including received headerframes 1304 and a receiver sample timeline 1302 (depicted below impulsesignal 1301 in FIG. 13) initially established by the impulse radio tosample the received header frames. As mentioned above, and as shown inFIG. 13, initially, the receiver sample timeline 1302 established by theimpulse radio receiver is neither frame synchronized nor codesynchronized with the received impulse signal 1301.

Therefore, because the impulse radio receiver is neither framesynchronized (i.e., frame aligned) nor short code synchronized it entersa timing acquisition mode to time synchronize sample timeline 1302 ofthe impulse radio receiver with the received impulse signal frames andthe short acquisition code of the received signal. The inventive processof achieving initial frame and short code synchronization is oftenreferred to as “first stage acquisition.”

As shown, exemplary received impulse signal 1301 includes consecutivelyreceived modulo-4 acquisition code sequences 1304. The exemplary shortacquisition code associated with signal 1301 is P0, P1, P2 and P3, isthe same as the exemplary short acquisition code discussed above inconnection with FIG. 12A. Each upward pointed arrow within a receivedframe of received signal 1301 represents an impulse within the receivedframe. The impulse is positioned with each frame according to the shortacquisition code used by the transmitter.

Initial receiver sample timeline 1302 includes four sample frames,including a zeroth sample frame SF0, a first sample frame SF1, a secondsample frame SF2 and a third sample frame SF3. The impulse radioreceiver initially samples the received signal 1301 in accordance withsample times (indicated by down arrows in timeline 1302) associated witheach of sample frames SF0-SF3. The impulse radio determines such sampletimes based on an apriori knowledge of the frame width and theacquisition code associated with the received impulse signal. As will beexplained below, when the receiver is both frame and code synchronizedwith the received signal, the sample times (indicated by the down arrowsin sample timeline 1302) will be aligned with the impulses (indicated bythe up arrows) in the received signal 1301. In other words, first stageacquisition achieves short code and frame synchronization.

According to exemplary initial receiver sample timeline 1302, theimpulse radio “attempts” to sample an impulse at a position P0 in zerothsample frame SF0, an impulse at a position P1 in first sample frame SF1,an impulse at a position P2 in second sample frame SF2, and an impulseat a position P3 in third sample frame SF3. As mentioned above, theimpulse radio receiver establishes these sample times based on theapriori knowledge of the frame width and the short acquisition codeassociated with the received impulse signal. The impulse radio receiver“attempts” to sample impulses because the sample times, at leastinitially, are merely guesses as to where impulses in received signal1301 are positioned. Therefore, it is likely the impulse radio receiverwill not successfully sample the impulses of a short code sequence 1204.

There are at least two reasons for the impulse radio receiver will notsuccessfully sample the impulses of a short code sequence 1204. First,the impulse radio receiver initially has not determined when in time thestart and end boundaries of the frames of received signal 1301 occur.Thus, it is likely there is a frame offset FO between the actualboundary of a received frame, and the frame boundary of initial sampleframe SF0 assumed by the impulse radio receiver, as shown in FIG. 13.Additionally, the impulse radio receiver initially does not haveknowledge of which frame of the short acquisition code sequence it isreceiving at a specific time. Thus, as shown in FIG. 13, the impulseradio receiver may be sampling position P0 (within SF0), when theimpulse in the received frame (i.e., the frame F1 concurrently beingreceived) is actually at position P1. This is due to what shall bereferred to as a code offset CO (and more specifically, in this case, ashort code offset CO).

As mentioned above, in the example of FIG. 13, each short acquisitioncode sequence includes four frames, wherein, as far as the receiver isconcerned, a pulse can be in one of four positions in each frame. If thebeginning 1306 of receiver sample timeline 1302 coincides with thebeginning of the zeroth frame FO of acquisition code sequence 1304, thenthe code offset CO equals zero (using the terminology set forth herein).If the beginning 1306 of receiver sample timeline 1302 coincides withthe first frame F1 of acquisition code sequence 1304, then the codeoffset CO equals one (i.e., CO=1). If the beginning 1306 of receiversample timeline 1302 coincides with the second frame F2 of acquisitioncode sequence 1304, then the code offset CO equals two (i.e., CO=2). Ifthe beginning 1306 of receiver sample timeline 1302 coincides with thethird frame F3 of acquisition code sequence 1304, then the code offsetCO equals three (i.e., CO=3). If the beginning 1306 of receiver sampletimeline 1302 coincides with the zeroth frame F0, there is no codeoffset (i.e., CO=0). Of course, if the short acquisition code had alength greater than 4, then additional possible code offsets wouldexist.

Put in other words, if the impulse radio is sampling the zeroth frame ofreceived signal 1301 when it should be sampling the third frame of theshort code (because that is where the impulse is located in the framebeing received), then the code offset equals three. If the impulse radiois sampling the zeroth frame when it should be sampling the second frameof the short code, then the code offset equals two. If the impulse radiois sampling the zeroth frame when it should be sampling the first frame,then the code offset equals one. If the impulse radio is correctlysampling the zeroth frame, then the code offset equals zero.

Even if there is a zero code offset, there can still be a frame offsetFO that causes the impulse radio receiver to fail to correctly samplethe impulses in the received frames. Thus, there is a need for theimpulse radio receiver to align itself (i.e., its timing) with both thereceive frames of received signal 1301 and the short acquisition codesequence 1304 of receive signal 1301. Stated another way, there is aneed to achieve proper frame alignment (i.e., frame synchronization) andcode synchronization. It is noted that first stage acquisition does notrequire that the frame offset be precisely zero. Rather, course framealignment is satisfactory for first stage acquisition. Thus, properframe alignment, as mentioned above, refers to such course framealignment. A tracking process that occurs following first stageacquisition will cause the frame offset to approach zero.

D. Overview of Solution to Problem

Acquisition in the present invention is divided into two phases,referred to as first stage acquisition and second stage acquisition.During the first stage an impulse radio receiver uses ramp builders(discussed in more detail below) to search for short acquisition codesequences of a packet header. When a specific threshold is satisfied,the transmitting impulse radio and receiving impulse radio are codealigned, to the extent of the short acquisition code, and coursely framealigned. An initial tracking process (i.e., “tracking”) can then beinitiated. Second stage acquisition is then used to locate thedelimeter, and thus, the end of the packet header, which is the same asthe beginning of the data payload. Demodulation of payload data beginsafter second stage acquisition. This method shall now be described withreference to FIG. 15.

FIG. 15 is a high level flow diagram illustrating a method 1500 foracquiring a pulse position modulated (ppm) impulse signal, according toan embodiment of the present invention.

Method 1500 begins at a step 1502 when the impulse radio receiverreceives (i.e., at least begins to receive) a ppm impulse signal packetthat includes a header followed by a data payload. The header includesrepeating short acquisition code sequences, which have been discussedabove. An exemplary header is discussed above in connection with FIG.12A. A delimiter of the header separates the header from the datapayload, as discussed in connection with FIG. 12A. The packet can alsoinclude one or more ratchet codes between the short acquisition codesequences and the delimiter, as discussed above in connection with FIG.12B, and described in more detail below.

At a next step 1504, course frame alignment is achieved based on a shortacquisition code (defined by a packet protocol) and short acquisitioncode sequences of the received impulse signal packet. Step 1504 isdiscussed in additional detail below in connection with FIG. 16. As willbe appreciated to one of ordinary skill in the art reading thisspecification, when frame alignment is achieved in accordance with thepresent invention using the short acquisition code, code alignment willalso be achieved to the extent of the short acquisition code. Step 1504,which is described in more detail below, represents first stageacquisition of the present invention.

Once frame aligned (and code aligned to the extent of the shortacquisition code) with the frames of the received signal, the impulseradio receiver can initiate tracking of the received impulse signaltiming at a step 1506. As will be explained in more detail below, thetime required to achieve frame alignment using the short acquisitioncodes in the present invention is significantly less than the timerequired using conventional longer acquisition codes. Thus, asignificant advantage of the present invention is that tracking can beinitiated very early during acquisition, thereby greatly improving theprobability for fast acquisition. Tracking of the received signal can beaccomplished, for example, using the tracking loop discussed inconnection with FIG. 11. The specific system and method of tracking thereceived signal is not important to the present invention. Rather, whatis significant to the present invention is that tracking can beinitiated very soon after the receiver receives the beginning of apacket, thereby providing for a better signal to noise ratio during theremainder of the acquisition process, thus improving the probability forfast acquisition. One of ordinary skill in the art will appreciate thatthere are various ways to track to the received signal. Even though itis preferable to initiate tracking once first stage acquisition isachieved, it is noted that many aspects of the present invention do notrely on beginning tracking at this instance. Accordingly, in alternativeembodiments of the present invention tracking can be initiated later inthe acquisition process.

At a next step 1508, the end of the header (which is the same as thebeginning of the data payload) is detected based on a delimeter and/orone or more ratchet codes (both defined by the packet protocol), and adelimeter sequence and/or one or more ratchet code sequences of thereceived impulse signal packet. This step can also achieve proper symboltiming, thus enabling subsequent data symbol demodulation. Step 1508,which is discussed in additional detail below in connection with FIGS.21 and 22, represents second stage acquisition of the present invention.

Finally, after the impulse radio receiver has achieved frame and shortcode synchronization (at step 1504), proper tracking (at step 1506), andrecovered symbol timing (at step 1508), the impulse radio receiverdetects data payload symbols at a step 1510. For example, the impulseradio receiver can demodulate (including decode) the ppm impulses of thedata payload at step 1510.

Additional details of demodulating ppm impulse signals are disclosed incommonly owned U.S. Pat. Nos. 4,641,317, 4,743,906, 4,813,057, and4,979,186; and U.S. Pat. No. 6,421,389, issued Jul. 16, 2002, entitled“Baseband Signal Converter Device for a Wideband Impulse RadioReceiver”; and copending U.S. patent application Ser. No. 09/537,264,entitled “System and Method Utilizing Multiple Correlator Receivers inan Impulse Radio System”; and copending U.S. patent application Ser. No.09/538,519, filed Mar. 29, 2000, entitled “Vector Modulation System andMethod for Wideband Impulse Radio Communications”; and U.S. patentapplication Ser. No. 09/537,692, filed Mar. 29, 2000, entitled,“Apparatus, System and Method for Flip Modulation in an Impulse RadioCommunication System,” each of which is incorporated herein by referencein its entirety.

E. Packet Protocol

Prior to describing additional details of the present invention, it isuseful to explain a protocol of the present invention. A packetprotocol, according to an embodiment of the present invention, definesat least each of the following:

-   -   a. the integration length, also referred to as the ramp interval        length N (e.g., N=16);    -   b. the length of the repeating short acquisition code 1204        (e.g., four frames);    -   c. the actual short acquisition code 1204 (e.g., P0, P1, P2,        P3);    -   d. the number of short acquisition code sequences per packet        (e.g., 40);    -   e. the length of each ratchet code sequence (e.g., 16 frames);    -   f. each actual ratchet code (e.g., P0, P1, P2, P3, P1, P2, P3,        P0, P2, P3, P0, P1, P3, P0, P1, P2);    -   g. the number of different unique ratchet code sequences        (e.g., 2) per packet;    -   f. the number of each different unique ratchet code sequence per        packet (e.g., 2);    -   g. the actual delimeter sequence (e.g., a flipped version of the        longest ratchet code sequence);    -   h. the long code used to encode the data payload; and    -   i. the length of the data payload (e.g., 1000 bits worth of        frames, which can equal 1000*N).

The packet protocol is known by impulse radios in communications withone another, such as an impulse radio transmitter and a cooperatingimpulse radio receiver. By knowing the short acquisition code used by atransmitter to produce repeating short code sequences of a header of animpulse signal packet, a receiving impulse radio (simply referred to asa receiver) can achieve frame alignment and code alignment to the extentof the short acquisition code. The receiver can then detect the end ofthe header (which is the same as the beginning of the data payload)based on one or more ratchet codes and/or a delimiter. All of this isexplained in detail below.

F. Frame Alignment (First Stage Acquisition)

FIG. 16 a illustrates additional details of step 1504 of method 1500,according to an embodiment of the present invention. It is noted thatthe step of achieving frame alignment is also referred to as a “firststage acquisition” because it is the first part of the acquisitionprocess (after a signal has been received). The steps discussed inconnection with FIG. 16 correspond to a method of achieving framealignment based on an exemplary modulo-4 acquisition code and repeatingmodulo-4 acquisition code sequences of the received signal. Codesynchronization is also achieved at step 1504 to the extent of the shortacquisition code, as shall be explained. One of ordinary skill in theart will appreciate how this method can be modified to achieve framealignment based on short acquisition codes of other lengths (e.g., amodulo-8 acquisition code, a modulo 16-acquisition code, etc.) afterreading the relevant descriptions provided herein.

Initially, the frame offset F0 and code offset of a received impulsesignal are unknown. As shown in FIG. 16 a, at a step 1602 a, a pluralityof frames (i.e., N frames, where N the integration length) of thereceived impulse signal packet are sampled based on the repeatingacquisition code (which is known by the impulse radio receiver), a frameoffset FO, and a code offset CO0 (i.e., CO=0), to thereby produce aplurality of samples corresponding to the frame offset FO and the codeoffset CO0. For example, if the known repeating acquisition code has alength of four frames (i.e., is a modulo-4 code), then the four (4)frames of the received impulse signal packet are sampled based on theknown acquisition code, the frame offset FO, and the code offset CO0.Concurrently (i.e., in parallel), at a step 1602 b, the same pluralityof frames of the received impulse signal packet are sampled based on therepeating acquisition code, the same frame offset FO, and a differentcode offset CO1 (i.e., CO=1), to thereby produce a plurality of samplescorresponding to the frame offset FO and the code offset CO1.Concurrently, at a step 1602 c, the same plurality of frames of thereceived impulse signal packet are sampled based on the repeatingacquisition code, the same frame offset FO, and a different code offsetCO2 (i.e., CO=2), to thereby produce a plurality of samplescorresponding to the frame offset FO and the code offset CO2.Concurrently, at a step 1602 d, the same plurality of frames of thereceived impulse signal packet are sampled based on the repeatingacquisition code, the same frame offset FO, and a different code offsetCO3 (i.e., CO=3), to thereby produce a plurality of samplescorresponding to the frame offset FO and the code offset CO3. (It isnoted that for this example, due to the lack of frame alignment, it ispossible that five frames, not just four frames, of the received impulsesignal packet are sampled at step 1602 a, 1602 b, 1602 c and/or 1602 d.)

At next parallel steps 1604 a, 1604 b, 1604 c and 1604 d, the pluralityof samples produced at steps 1602 a, 1602 b, 1602 c and 1602 d,respectively, are accumulated to produce ramp values (also referred toas accumulated values). More specifically, at step 1604 a, the pluralityof samples corresponding to the frame offset FO and the code offset CO0are accumulated (e.g., by a ramp builder) to produce a ramp value RV0.Concurrently, at step 1604 b, the plurality of samples corresponding tothe frame offset FO and the code offset CO1 are accumulated (e.g., by aseparate ramp builder) to produce a ramp value RV1. Concurrently, atstep 1604 c, the plurality of samples corresponding to the frame offsetFO and the code offset CO2 are accumulated (e.g., by another rampbuilder) to produce a ramp value RV2. Concurrently, at step 1604 d, theplurality of samples corresponding to the frame offset FO and the codeoffset CO3 are accumulated (e.g., by another ramp builder) to produce aramp value RV3. The ramp values may be stored so that they can be usedtogether with concurrently and/or later generated ramp values in athreshold determination.

At step 1605, discriminants are calculated from the ramp values to beused in a selected threshold equation. These discriminants are shown inFIG. 16 b.

At a next step 1606, there is a determination of whether a threshold(e.g., a first state acquisition threshold) has been satisfied. Thisdetermination is based on one or more of ramp values RV0, RV1, RV2 andRV3. Various methods for performing this threshold determination,according to the present invention, are discussed in detail below.

If the threshold has been satisfied (i.e., if the answer to step 1606 isYES), then the impulse radio receiver determines that it has identifiedthe correct frame time offset and code offset, and thus, the impulseradio receiver has achieved frame alignment and short acquisition codesynchronization. Flow then proceeds to step 1506, which has beendiscussed above, and tracking is initiated.

Failure to satisfy the threshold will typically indicate that the sampleframes are not frame aligned with the frames of the received packetbecause all code offsets have been searched (leaving only framemisalignment as the reason for not meeting the threshold). Accordingly,failure to satisfy the threshold shall be interpreted as indicating thatthere is an unacceptable offset between the beginning of each frame inthe received packet and the beginning of each frame of the sampleframes. Thus, if the threshold has not been satisfied (i.e., if theanswer to step 1606 is NO), then the frame offset FO is adjusted by astep size (e.g., FO=FO+3 nsec, where 3 nsec is the step size) at a step1608 so that another frame offset can be tested using the short codesearch steps 1602-1606.

In other words, at steps 1602(a-d), 1604(a-d) and 1606, each of thepossible code offsets (four, in this example, i.e., CO0, CO1, CO2 andCO3) are essentially analyzed in parallel. If the threshold is notsatisfied, it is assumed that the reason is because there exists anunacceptable frame offset FO. Thus, at step 1608, the frame offset isadjusted so that another frame offset can be tested.

After the frame offset FO has been adjusted at step 1608, flow returnsto steps 1602 a, 1602 b, 1602 c and 1602 d where additional frames ofthe repeating code sequence are sampled using the new frame offset FO.This process (i.e., steps 1602-1608) is repeated, each time with a newframe offset FO, until the threshold is satisfied, thereby indicatingthat the impulse radio receiver has achieved satisfactory framealignment (also referred to as frame synchronization), and also shortacquisition code synchronization.

Parallel steps 1602 a, 1602 b, 1602 c and 1602 d, and parallel steps1604 a, 1604 b, 1604 c and 1604 d can be explained with reference to theexample of FIGS. 17 and 18. Shown at the top of FIG. 17 is a portion ofa header of a received impulse signal 1701 that includes a repeatingmodulo-4 acquisition code sequence 1704. The exemplary repeatingacquisition code sequence has a hop sequence P0, P1, P2, P3. Fourreceiver time-lines 1702 a, 1702 b, 1702 c and 1702 d are shown, eachstarting at an initial sample frame time 1706. As shown in this example,the initial sample frame time 1706 is offset from the closest (in time)frame boundary of received signal 1701 by a frame offset FO, where FO_0.

The receive timeline 1702 a begins with (i.e., the initial sample frameSF0 includes) an impulse at the same position as the impulse in receiveframe F0 of acquisition code sequence 1704, and thus, receive timeline1702 a has a zero code offset. Receiver timeline 1702 a is useful forexplaining step 1602 a, where a plurality of frames of received impulsesignal 1701 are sampled based on:

-   -   1. the repeating acquisition code (having the hop sequence P0,        P1, P2, P3, which is known);    -   2. the frame offset FO; and    -   3. a code offset CO0 (code offset=0),    -   to thereby produce a plurality of samples corresponding to frame        offset FO and code offset CO0. As discussed above, downward        pointed arrows represent the sample times within sample frames        SF0, SF1, SF2 and SF3.

As indicated by the arrows pointing to the accumulation block (í), theplurality of samples (e.g., four samples) corresponding to the frameoffset FO and the code offset CO0 are accumulated, at step 1604 a, toproduce a ramp value RV0. Since, in the example, none of the sampletimes of receiver timeline 1702 a coincide with the impulses(represented by upward pointed arrows) of received impulse signal 1701,the four amplitude values (accumulated to produce RV0) are notindicative of the actual received impulse energy of received impulsesignal 1701. However, it is likely that the four amplitude values areindicative of received noise and/or interference. It is also possiblethat the four amplitude values are indicative of ring down from thereceived impulses.

The next receive timeline 1702 b begins with (i.e., the initial sampleframe SFO includes) an impulse at the same position as the impulse inreceive frame F1 of acquisition code sequence 1704, and thus, receivetimeline 1702 b has a code offset equal to one. Receiver timeline 1702 bis useful for explaining step 1602 a, where a plurality of frames ofreceived impulse signal 1701 are sampled based on:

-   -   1. the repeating acquisition code (having the hop sequence P0,        P1, P2, P3, which is known);    -   2. the frame offset FO, and    -   3. a code offset CO1 (code offset=1),    -   to thereby produce a plurality of samples corresponding to frame        offset FO and code offset CO1. Because the code offset equals        one, the hop sequence of sample frames SF0-SF3 of receive        timeline 1702 b is P1, P2, P3, P0, because the code sequence is        rotated one position.

As indicated by the arrows pointing to the accumulation block (í), theplurality of samples corresponding to the frame offset FO and the codeoffset CO1 are accumulated, at step 1604 b, to produce a ramp value RV1.Since none of the sample times of receiver timeline 1702 b are alignedwith the impulses (represented by upward pointed arrows) of receivedimpulse signal 1701, the four amplitude values (accumulated to produceRV1) are not indicative of the received impulse energy of receivedimpulse signal 1701. However, it is likely that the four amplitudevalues are indicative of received noise, interference, and/or ring downfrom the received impulses.

The next receive timeline 1702 c begins with (i.e., the initial sampleframe SFO includes) an impulse at the same position as the impulse inreceive frame F2 of acquisition code sequence 1704, and thus, receivetimeline 1702 c has a code offset equal to two. Receiver timeline 1702 cis useful for explaining step 1602 c, where a plurality of frames ofreceived impulse signal 1701 are sampled based on:

-   -   1. the repeating acquisition code (having the hop sequence P0,        P1, P2, P3, which is known);    -   2. the frame offset FO, and    -   3. a code offset CO2 (code offset=2),    -   to thereby produce a plurality of samples corresponding to frame        offset FO and code offset CO2. Because the code offset equals        two, the hop sequence of sample frames SF0-SF3 of receive        timeline 1702 c is P2, P3, P0, P1 (i.e., the code sequence is        rotated two positions).

As indicated by the arrows pointing to the accumulation black (í), theplurality of samples corresponding to the frame offset FO and the codeoffset CO2 are accumulated, at step 1604 c, to produce a ramp value RV2.Since none of the sample times of receiver timeline 1702 c are alignedwith the impulses (represented by upward pointed arrows) of receivedimpulse signal 1701, the four amplitude values (accumulated to produceRV2) are not indicative of the received impulse energy. However, it islikely that the four amplitude values are indicative of received noise,interference, and/or ring down from the received impulses.

The receive timeline 1702 d begins with (i.e., the initial sample frameSFO includes) an impulse at the same position as the impulse in receiveframe F3 of acquisition code sequence 1704, and thus, receive timeline1702 c has a code offset equal to three. Receiver timeline 1702 c isuseful for explaining step 1602 d, where a plurality of frames ofreceived impulse signal 1701 are sampled based on:

-   -   1. the repeating acquisition code (having the hop sequence P0,        P1, P2, P3, which is known);    -   2. the frame offset FO, and    -   3. a code offset CO3 (code offset=3),    -   to thereby produce a plurality of samples corresponding to frame        offset FO and code offset CO3. Because the code offset equals        three, the hop sequence of sample frames SF0-SF3 of receive        timeline 1702 c is P3, P0, P1, P2 (i.e., the code sequence is        rotated three positions).

As indicated by the arrows pointing to the accumulation block (í), theplurality of samples corresponding to the frame offset FO and the codeoffset CO3 are accumulated, at step 1604 d, to produce a ramp value RV3.In this instance, one of the four sample times of receiver timeline 1702d are coincidentally aligned with an impulses of received impulse signal1701. Thus, the one of the four amplitude values accumulated to produceRV3 are indicative of the received impulse energy (i.e., the sample timein SF2 coincides with the impulse in the fourth shown frame of impulsesignal 1701). The four amplitude values can also be indicative ofreceived noise and/or interference, and/or ring down from the receivedimpulses.

One of the purposes of the threshold determination is to recognize thatcode and frame offset synchronization has not occurred when, forexample, just one of the four sample times of receiver timeline 1702 dare aligned with an impulses of received impulse signal 1701. That is,the threshold detection is used to prevent false indications of frameand code synchronization. Various threshold determination embodimentsare discussed below.

Assuming the impulse radio receiver determines that the threshold is notsatisfied (i.e., at steps 1606), the frame offset is adjusted at step1608. After the frame offset FO has been adjusted at step 1608, flowreturns to steps 1602 a, 1602 b, 1602 c and 1602 d where additionalframes of the repeating short code sequence are sampled using the newframe offset FO. This process (i.e., steps 1602-1608) is repeated, eachtime with a new frame offset FO, until the threshold is satisfied,thereby indicating that impulse radio receiver has achieved proper framealignment and code synchronization to the extent of the acquisitionshort code. FIG. 18, which will now be explained in detail, illustratesthe situation where there exists essentially no frame offset, causingthe threshold to be satisfied.

Referring to FIG. 18, four receiver time-lines 1802 a, 1802 b, 1802 cand 1802 d are shown, each starting at an initial sample frame time1806. As shown in this example, there is essentially no offset betweenthe initial sample frame time 180 and the closest (in time) frameboundary of received signal 1701 (i.e., frame offset FO≈0).

The receive timeline 1802 a begins with (i.e., the initial sample frameSFO includes) frame F0 of acquisition code sequence 1704, and thus,receive timeline 1802 a has a zero code offset. Receiver timeline 1802 ais useful for explaining step 1602 a, where a plurality of frames ofreceived impulse signal 1701 are sampled based on the repeatingacquisition code (having the hop sequence P0, P1, P2, P3, which isknown), the frame offset FO≈0, and a code offset CO0 (code offset=0), tothereby produce a plurality of samples corresponding to frame offsetFO≈0 and code offset CO0. As discussed above, downward pointed arrowsrepresent the sample times within sample frames SF0, SF1, SF2 and SF3.

As indicated by the arrows pointing to the accumulation block (í), theplurality of samples corresponding to the frame offset FO≈0 and the codeoffset CO0 are accumulated, at step 1604 a, to produce a ramp value RV0.Since none of the sample times of receiver timeline 1802 a coincide withthe impulses (represented by upward pointed arrows) of received impulsesignal 1701, the four amplitude values (accumulated to produce RV0) arenot indicative of the actual received impulse energy of received impulsesignal 1701. However, it is likely that the four amplitude values areindicative of received noise and/or interference. It is also possiblethat the four amplitude values are indicative of ring down from thereceived impulses.

Similarly, the amplitude values accumulated to produce ramp value RV2(associated with receive timeline 1802 c), and the amplitude valuesaccumulated to produce ramp value RV3 (associated with receive timeline1802 d), are not indicative of the actual received impulse energy ofreceived impulse signal 1701. However, as explained immediately below,the amplitude values accumulated to produce ramp value RV1 (associatedwith receive timeline 1802 b) are indicative of the actual receivedimpulse signal energy of received impulse signal 1701.

The receive timeline 1802 b begins with (i.e., the initial sample frameSFO includes) frame F1 of acquisition code sequence 1704, and thus,receive timeline 1802 b has a code offset equal to one. Receivertimeline 1802 b is useful for explaining step 1602 a, where a pluralityof frames of received impulse signal 1701 are sampled based on therepeating acquisition code (having the hop sequence P0, P1, P2, P3,which is known), the frame offset FO≈0, and a code offset CO1 (codeoffset=1), to thereby produce a plurality of samples corresponding toframe offset FO≈0 and code offset CO1. Because the code offset equalsone, the hop sequence of sample frames SF0-SF3 of receive timeline 1802b is P1, P2, P3, P0 (i.e., the code sequence is rotated one position).

As indicated by the arrows pointing to the accumulation block (í), theplurality of samples corresponding to the frame offset FO=0 and the codeoffset CO1 are accumulated, at step 1604 b, to produce a ramp value RV1.Since all of the sample times of receiver timeline 1802 b are alignedwith the impulses (represented by upward pointed arrows) of receivedimpulse signal 1701, each of the four amplitude values (accumulated toproduce RV1) are indicative of the received impulse energy of receivedimpulse signal 1701. Additionally, it is likely that the four amplitudevalues are also indicative of received noise, interference, and/or ringdown from the received impulses.

In this instance, it will be determined at step 1606, that the thresholdhas been satisfied. Threshold detection is discussed in detail below.Satisfaction of the threshold signifies that frame synchronization hasbeen acquired. In this example, satisfaction of the threshold alsosignifies that the actual code offset is equal to one (i.e., codeoffset=1, or CO1), thereby enabling the impulse radio receiver toachieve code synchronization to the extent of the short acquisitioncode.

Once frame alignment and short acquisition code synchronization areachieved, the impulse radio receiver can begin to track the receivedsignal using a tracking loop. An exemplary tracking loop is discussedabove in connection with FIG. 11. As mentioned above, the specificsystem and method of tracking the received signal is not important tothe present invention. Rather, what is significant to the presentinvention is that tracking can be initiated very early in theacquisition process, thereby improving a signal-to-noise (S/N) ratioduring the remainder of the acquisition process, thus improving theprobability for fast acquisition. One of ordinary skill in the art willappreciate that there are various ways to track to the received signal.Even though it is preferable to initiate tracking once first stageacquisition is achieved, it is noted that many aspects of the presentinvention do not rely on tracking beginning at this instance.Accordingly, in alternative embodiments of the present inventiontracking can be initiated later in the acquisition process.

Following the tracking and/or in parallel with the tracking, the impulseradio receiver searches for the end of the header (which is the same asthe beginning of the data payload). Additionally, the impulse radioreceiver determines proper symbol timing. These features are discussedbelow.

In the above discussed example, each accumulated ramp value (e.g., RV0,RV1, RV2 and RV3) was the sum of four (4) amplitude samples of impulsesignal packet 1202. Thus, the ramp integral length (also referred to asintegration length) was four (4). It is much more likely that theintegration length is an integer multiple of the length of the shortacquisition code (e.g., 8, 16, 32, 64, 256, 512, etc.), to achieve animproved S/N ratio due to integration. The above described step ofachieving code and frame synchronization (i.e., step 1504) isessentially the same when the integration length is greater, except thatthe threshold determination step (e.g., 1606) would be based onaccumulated ramp values that are the sum (i.e., integration) of moresamples. For example, if the integration length is eight (8), then thethreshold determination step would occur once every 8 frames (ratherthan once every four frames as described above) and each accumulatedramp value would be the sum of 8 samples. Similarly, if the integrationlength is two hundred and fifty six (256), then the thresholddetermination step would occur once every 256 frames and eachaccumulated ramp value would be the sum of 256 samples.

G. Threshold Determination

Various embodiments relating to step 1606, the step of determiningwhether a threshold has been satisfied, shall now be discussed. In eachof these embodiments, this determination is based on ramp values (e.g.,RV0, RV1, RV2 and/or RV3), which are determined at step 1604 (e.g., 1604a-1604 d) and threshold discriminants, which are determined in steps1605. One of ordinary skill in the art will understand that otherthreshold equations constructed from the threshold discriminants arewithin the sprit and scope of the present invention. The equations usedto decide when first stage acquisition is complete are called the firststage acquire threshold equations. Simulations have identified severalpossible threshold equations involving the following quantities setforth in the table (Table 1) below:

TABLE 1 Threshold Equaiton Quantities NumberN Quantity Description 11MaxR 1st largest acquisition ramp NextR 2nd largest acquisition rampMaxV Maximum correlator variance MinV Minimum correlator variance MeanVMean correlator variance MA_MeanV Moving (historical) average of themean correlator variance MA_MinV Moving (historical) average of theminimum correlator variance Logic_1 Constant value 1 Logic_0 Constantvalue 0

Using the description and quantity information above, a basic set offirst stage acquire threshold equations are given in the table (Table2A) below.

TABLE 2A Threshold Equations NumberN Number Threshold EquationholdEquation 11 1 N * MaxR/MA_meanV > C 2 MaxR/NextR > C 3 Logic AND or ORof equation 1 and 2 (via the Master Sequencer) 4 N * MaxR/MaxV > C 5 N *(MaxR/MA_meanV) * (MinV/MaxV) > C 6 N * (MaxR/MA_meanV) *[FORCE_ZERO(MinV − MA_MinV)/ FORCE_ONE(MaxV − MA_MinV)] > C 7 N * (MaxR− NextR)/MA_meanV > C 8 N * [(MaxR − NextR)/MA_meanV] * (MinV/ MaxV) > C

Further, the following equations can be used, which are similar to theabove, however, are greater than or equal to. To wit (Table 2B):

TABLE 2B Threshold Equations NumberN Number Threshold EquationholdEquation 11 1 N * MaxR/MA_meanV ≧ C 2 MaxR/NextR ≧ C 3 Logic AND or ORof equation 1 and 2 (via the Master Sequencer) 4 N * MaxR/MaxV ≧ C 5 N *(MaxR/MA_meanV) * (MinV/MaxV) ≧ C 6 N * (MaxR/MA_meanV) *[FORCE_ZERO(MinV − MA_MinV)/ FORCE_ONE(MaxV − MA_MinV)] ≧ C 7 N * (MaxR− NextR)/MA_meanV ≧ C 8 N * [(MaxR − NextR)/MA_meanV] * (MinV/ MaxV) ≧ C

It is understood that other equations can be realized using up to fourthreshold equation setup registers. In the equations above, N representsthe acquisition integration length and C is some programmable constant.The FORCE_ZERO(x) function yields x for x≧0 and 0 for x<0 and theFORCE_ONE(x) function yields x for x≧1 and 1 for x<1.

To provide a flexible thresholding scheme, the acquisition logicincludes four fully programmable threshold equations. The equations havethe following format.

Register1*Term1*OP1(Term2−Term3)<,>,=,≠ Register2*Term4*OP2(Term5−Term6)

Each of the four threshold equations can be programmed such that terms1-6 can select any of the quantities in the table of “Important FirstStage Acquisition Quantities”. The logic operators, OP1 and OP2, can beset to FORCE_ONE, FORCE_ZERO or NOP (no operation). Register1 andRegister2 are 12-bit user configurable registers that realize the N andC constants in the equations above. All of the equations listed in thetable of “Example First Stage Acquire Threshold Equations” (as well asmany others) can be realized. Also, to provide a flexible comparisonscheme, the equations left and right side may be compared using thegreater than, less than, equal to, or not equal to operators. Theequations in the table of “First Stage Acquire Threshold Equations”currently only require the greater than or greater than or equal tocomparison.

To support equation number 3 in the table of “First Stage AcquireThreshold Equations”, the Boolean result of each threshold equation maybe combined using logic AND or logic OR in the Master Sequencer.

It should be emphasized that the programmable threshold equation logicis only used during short and long code FSA. Second stage acquisitionuses separate threshold logic.

The first stage acquisition threshold equation logic is shown in FIG. 16c at 1620, wherein selectable term quantities are input at 1622, 1624,1626, 1628 and 1630 and 1632. At 1634 Term 1 Mux is selected, at 1636Term 2 Mux is selected, at 1638 Term 3 Mux is selected, 1640 Term 4 Muxis selected, at 1642 Term 5 Mux is selected and at 1644 Term 6 Mux isselected.

At 1646 Term 2 Mux and Term 3 Mux are substracted and output as OP1 at1650, and at 1648, Term 5 Mux and Term 6 Mux are subtracted and outputas OP2 at 1652. At 1654 Term 1 Mux is multiplied by OP1 and at 1656 Term4 Mux is multiplied by OP2. Register1 at 1658 is multiplied by theoutput of 1654 at 1660. Register2 at 1662 is multiplied by the output of1656 at 1664. The output of 1660 and 1664 are compared at 1666 at outputat 1668.

The threshold determination techniques just set forth are further setforth in co-pending, patent application entitled, “SYSTEM AND METHOD FORPROCESSING SIGNALS IN UWB COMMUNICATIONS”, Ser. No. 10/712,269, filed on14 Nov. 2003 by Brethour et al. This patent application is incorporatedherein by reference in its entirety and for all purposes.

One of ordinary skill in the art will also understand that the multipleramp values used in the present invention might be built by a singleramp builder.

Another possible method for determining whether a threshold has beensatisfied is comparing each of the acquisition ramp values (e.g., RV0,RV1, RV2 and/or RV3) to a predetermined threshold value (i.e., a firststage acquisition threshold value). However, this method is not optimumbecause one of the ramps can exceed the predetermined threshold valuedue to noise and/or interference, thereby causing an impulse radioreceiver to falsely determine that frame and code synchronization hasbeen acquired. Further, noise and/or interference can cause more thanone of the ramps to be greater than the predetermined threshold value.Accordingly, in specific embodiments of the present invention discussedbelow, threshold determinations are based on relative comparisonsbetween the acquisition ramp values, rather than comparing theacquisition ramp values to a static predetermined value.

In a first embodiment of the present invention, the first largestacquisition ramp (also referred to as MaxR) is compared to the secondlargest acquisition ramp (also referred to NextR). More specifically,the threshold is satisfied if the first largest acquisition ramp is apredetermined amount of times greater than the next largest acquisitionramp (i.e., MaxR/NextR>C). In one example, the threshold is satisfied ifthe first largest acquisition ramp is more than three (i.e., C=3) timesgreater than the next largest acquisition ramp (i.e., MaxR/NextR>3). Oneof ordinary skill in the art would understand that other values for Care within the spirit and scope of the present invention.

A relative comparison between the first largest acquisition ramp and thenext largest acquisition ramp (as done in this embodiment) hasadvantages over a mere comparison of the first largest acquisition rampto a predetermined value. First of all, a determination that the firstlargest acquisition ramp (MaxR) is more than a predetermined amount oftimes (e.g., 3 times) greater than the next largest acquisition ramp(NextR) also means that the first largest acquisition ramp (MaxR) ismore than the predetermined amount of times (e.g., 3 times) greater thaneach of the other acquisition ramps. This gives a high level confidencethat there is not a false threshold detection as can occur when the rampvalues are compared to a predetermined threshold. Further, it is likelythat noise and/or interference can cause the first largest acquisitionramp to be greater than a predetermined value, thereby causing animpulse radio receiver to falsely determine that frame and codesynchronization has been acquired (i.e., if the threshold determinationwas based on a comparison to a static predetermined value). In contrast,because it is likely that noise and/or interference will corrupt each ofthe ramps in a relatively similar manner, it is not likely that suchnoise and/or interference can cause the first largest acquisition rampto be more than three (3) times greater than the next largestacquisition ramp. This provides a further level confidence that there isnot a false threshold detection. Further, it is likely that directcurrent (DC) offsets can cause the first largest acquisition ramp valueto be greater than a predetermined value, thereby causing an impulseradio receiver to falsely determine that frame and code synchronizationhas been acquired (i.e., if the threshold determination was based on acomparison to a static predetermined value). In contrast, because it islikely that DC offsets will corrupt each of the ramps in a relativelysimilar manner, it is not likely that such DC offsets can cause thefirst largest acquisition ramp to be more than three (3) times greaterthan the next largest acquisition ramp. Therefore, relative comparisonsbetween ramp values have advantages over against static predeterminedthresholds.

In a second embodiment of the present invention, whether a threshold hasbeen satisfied can be determined according to the following equation:

N·MaxR/(MA MeanV)≧C

-   -   where,    -   N—is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR—is the first largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of a mean variance        (associated with ramps or positions); and    -   C—is a predetermined constant (e.g., 3).

The variance associated with a ramp is the variance of the samples(i.e., the N samples) accumulated to produce the ramp. Thus, forexample, if four ramps are produced, then the “mean variance” can be theaverage of four variance values (where each variance value is associatedwith a different one of the four ramps).

As is apparent from FIG. 18, every possible position (e.g., P0, P1, P2and P3) within a sample frame (e.g., SF0) is sampled during parallelsteps 1602 (e.g., 1602 a-1602 d). The variance associated with apossible position (e.g., P0, P1, P2 or P3) is the variance of thesamples (i.e., N samples) at one of the positions. Thus, for example, ifan impulse can be located at one of four possible positions within eachsample frame, then the mean variance can be the average of four variancevalues (where each variance value is associated with a differentpossible position).

The well known equation for variance is:

$\sigma^{2} = \frac{\sum\limits_{i = 1}^{N}\left( {x_{i} - \mu} \right)^{2}}{N}$

In this example,

-   -   σ² represents the amplitude variance of multiple samples (either        associated with a ramp, or a possible position),    -   xi represents the amplitude of one of multiple samples,    -   N represents the number of multiple samples used in determining        the variance (this can be the same as the acquisition ramp        integral length, but may be more), and    -   μ represent the mean (i.e., average) amplitude of the multiple        data samples (associated with a ramp, or a possible position).

The above equation determines a biased amplitude variance. Other typesof amplitude variance that can be used include an unbiased samplevariance (where the denominator is N−1) and an absolute variance. One ofordinary skill in the art will appreciate that additional measures ofvariance can also be used. Additionally, one of ordinary skill in theart will understand how to determine the moving (historical) average ofthe mean variance.

In a third embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

N·(MaxR/MaxV)≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   MaxV is the maximum variance (associated with ramps, or        positions, as explained above); and    -   C is a predetermined constant (e.g., 3).

In a fourth embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

{N≠(MaxR/MA mean V)·(MinV/MaxV)}≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of the mean        variance (associated with ramps, or positions, as explained        above);    -   MIN V is the minimum variance (associated with ramps, or        positions, as explained above);    -   MaxV is the maximum variance (associated with ramps, or        positions, as explained above); and    -   C is a predetermined constant (e.g., 3).

In a fifth embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

{N·(MaxR/MA mean V)·[(MaxR−NextR)/MaxR)]}≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of the mean        variance (associated with ramps or positions, as explained        above);    -   NextR is the second largest acquisition ramp value; and    -   C is a predetermined constant (e.g., 3).

In a sixth embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

{N·(MaxR/MA mean V)·[FORCE_(—) POS(MinV−MA MinV)/FORCE_ONE(MaxV−MAMinV)]}≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of the mean        variance (associated with ramps, or positions, as explained        above);    -   MA Min V—is the moving (historical) average of the minimum        variance (associated with ramps, or positions, as explained        above);    -   MA Max V—is the moving (historical) average of the maximum        variance (associated with ramps, or positions, as explained        above);    -   MIN V is the minimum variance (associated with ramps, or        positions, as explained above);    -   MAX V is the maximum variance (associated with ramps, or        positions, as explained above)    -   The FORCE_POS(x) function yields x for x≧0, and 0 for x<0;    -   The FORCE_ONE(x) function yields x for x≧1, and 1 for x<1; and    -   C is a predetermined constant (e.g., 3).

In an seventh embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

{N·(MaxR−NextR)/MA mean V}≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of the mean        variance (associated with ramps, or positions, as explained        above);    -   NextR is the second largest acquisition ramp value; and    -   C is a predetermined constant (e.g., 3).

In an eighth embodiment, whether a threshold has been satisfied can bedetermined according to the equation:

{N·[(MaxR−NextR)/MA mean V]·(MinV/MaxV)}≧C

-   -   where,    -   N is the acquisition ramp integral length (i.e., the number of        samples accumulated in each ramp), a multiple of the acquisition        ramp integral length (i.e., N*2), or a defined constant (i.e.,        N=64);    -   MaxR is the first largest acquisition ramp value;    -   NextR is the second largest acquisition ramp value;    -   MA Mean V—is the moving (historical) average of the mean        variance (associated with ramps, or positions, as explained        above);    -   Min V is the minimum variance (associated with ramps, or        positions, as explained above);    -   Max V is the maximum variance of (associated with ramps, or        positions, as explained above); and    -   C is a predetermined constant (e.g., 3).

In a ninth embodiment, which is a combination of any two of the aboveembodiments, a threshold is satisfied when the conditions of bothspecified equations are satisfied (e.g., (MaxR/NextR≧C) AND (N·MaxR/MAMeanV≧C)) or when the conditions of either of the two equations aresatisfied (e.g., (MaxR/NextR≧C) OR (N·MaxR/MA MeanV≧C)).

Discussed above are the preferred embodiments for determining whether athreshold has been satisfied. Each of these embodiments reduces thechance of a false threshold detection resulting from noise, interferenceand/or DC offset. One of ordinary skill in the art would understand thatother threshold determinations are within the spirit and scope of thepresent invention.

H. Tracking

First stage acquisition of the present invention provides for fast andefficient frame alignment and code synchronization to the extent of theshort acquisition code, as described above. Accordingly, the presentinvention provides for a very early opportunity to track the receivedsignal. Proper tracking enables the oscillator in the receiving impulseradio to lock onto the oscillator in the transmitting impulse radio sothat the oscillators are not drifting with respect to one another. Thiswill result in an impulse that holds still within a sampling window ofthe receiving impulse radio. One of ordinary skill in the art willappreciate the benefits of tracking a signal as early as possible in theacquisition process. For example, proper tracking of the received signalimproves the signal to noise ratio (S/N) of samples of that signalbecause received impulses can be more precisely sampled in time.

Tracking can be accomplished in the manner discussed above in connectionwith FIGS. 6, 7 and 11. Alternatively, tracking can be accomplishedusing any tracking method known in the relevant art. It is notnecessarily the precise method of tracking that is unique to the presentinvention (although, as discussed below, a unique method is preferablyused). Rather it is the extremely early opportunity to track a signalthat is unique to the present invention. As mentioned above, even thoughit is preferable to initiate tracking once first stage acquisition isachieved, it is noted that many aspects of the present invention do notrely on tracking beginning at this instance. Accordingly, in alternativeembodiments of the present invention, tracking can be initiated later inthe acquisition process (e.g., during or following second stageacquisition).

In a preferred embodiment of the present invention, what are referred toas “back ramp values” are used during tracking. This will be explainedin more detail below. However, this use of “back ramp values” is notnecessary to accomplish many other aspects of the present invention, aswill be appreciated by one of ordinary skill in the art.

I. Back Ramps

Once first stage acquisition is complete, the impulse radio receiversearches for the appropriate ratchet code(s) (as part of second stageacquisition) in a manner described below. As shall be explained below,the impulse radio receiver begins this process by sampling the receivedsignal in accordance with the first expected ratchet code. It is verylikely that the impulse radio receiver will sample the received signalin accordance with the first expected ratchet code while the receivedpacket still consists of the repeating short acquisition code sequences.This occurs when the receiver completes first stage acquisition usingless than all of the repeating short acquisition code sequences of thepacket (e.g., if first stage acquisition is achieved using 32 of 40repeating short acquisition code sequences). During this intermediateperiod, there is the possibility that tracking of the received signal isadversely affected because the tracking loop may operate in a partialopen loop fashion (e.g., because the tracker may not be tracking impulsesignal energy because the radio may not be sampling actual impulses).This can reduce the S/N ratio of samples, and may even cause theoscillator in the impulse radio receiver to drift (with respect to theoscillator of the impulse radio transmitter) to the extent thatacquisition of the packet is lost.

Accordingly, in an embodiment of the present invention, ramp valuescontinue to be built based on the short acquisition code, during secondstage acquisition. In one embodiment of the present invention, whenfirst stage acquisition is complete, the detected short acquisition codeis loaded into a “back” register or a back sequence generator, andsamples generated in accordance with the short acquisition code are usedfor tracking, instead of samples generated in accordance with a ratchetcode used in second stage acquisition. These ramps being built accordingto the short acquisition code, during second stage acquisition, arereferred to herein as “back ramps”, since they are produced in abackground process during tracking and/or second stage acquisition (aforeground process). The ramp builder(s) used to build ramp valuesaccording to the short acquisition code, during second stageacquisition, is referred to a “back ramp”, since it is operating in thebackground during tracking and/or second stage acquisition.

In accordance with an embodiment of the present invention, the smallestback ramp value (BRV) is stored in a memory or register so that it canbe used for a threshold determination(s) of second stage acquisition.Each time a smaller BRV is determined, it replaces the previously storedsmallest BRV. In another embodiment of the present invention, a largestBRV is stored. In still another embodiment, an average of many BRVs isdetermined and stored. In a further embodiment of the present invention,the last BRV generated during tracking, but prior to second stageacquisition, is stored. The use of such BRVs will be described below inthe discussion of second stage acquisition.

J. Detect the Beginning of Data Payload (Second Stage Acquisition)

As mentioned above, each packet preferably includes a delimiter that isused to indicate the end of the header and the beginning of the datapayload. As explained in method 1500, after frame alignment (and codesynchronization to the extent of the short acquisition code) has beenachieved (at step 1504), and tracking has been initiated (at step 1506),the next step is to detect the beginning of the data payload, which isthe same as the end of the header (at step 1508). According to anembodiment of the present invention, this is accomplished by detecting adelimiter that defines the end of the packet header.

An exemplary packet was discussed above in connection with FIG. 12A.Referring again to FIG. 12A, packet 1202 includes header portion 1203(also referred to as header 1203) followed by data payload portion 1208(also referred to as data payload 1208). Header 1203 includes aplurality of acquisition short code sequences 1204 (also referred to asa plurality of short codes 1204) followed by delimiter 1206.Accordingly, the “delimiter” refers to the one or more frames in thepacket used to indicate the end of the header and the beginning of thedata payload.

As mentioned above, in one embodiment, delimiter 1206 includes the samenumber of frames as acquisition code sequence 1204. Thus, if theacquisition short code repeats every four frames (i.e., is a modulo-4acquisition code), as shown in FIG. 12A, then delimiter 1206 is alsofour (4) frames in length. In another embodiment, described in moredetail below, delimiter 1206 includes an integer multiple of the numberof frames included in the acquisition code sequence 1204. Moregenerally, as will be explained below, the length of the delimiter ispreferably the same as the ramp integral length N (i.e., the number ofsamples accumulated in each ramp). The integration length (also referredto as the ramp interval length) is selected such that an acceptablesignal to noise ratio can be achieved. Preferably, the integrationlength is defined by the packet protocol of the present invention.

The embodiment where delimiter 1206 includes the same number of framesas the repeating acquisition code sequence is only used if integrationgain is not being used by the transmitter and receiver to improve thesignal to noise ratio. This embodiment can be accomplished by making thedelimiter identical to the acquisition code sequence, except that eachimpulse is flipped. Thus, based on the example shown in FIG. 12A, thedelimiter would include four frames, with a flipped impulse in positionP0 in the frame F0, a flipped impulse in position P1 in frame F1, aflipped impulse in position P2 in frame F2, and a flipped impulse inposition P3 in frame F3. As mentioned above, this is illustrated in FIG.14, which shows an exemplary acquisition code sequence 1204 and itscorresponding delimiter 1206. Systems and methods for producing flippedimpulses are disclosed in commonly assigned co-pending U.S. patentapplication Ser. No. 09/537,692, filed Mar. 29, 2000, entitled,“Apparatus, System and Method for Flip Modulation in an Impulse RadioCommunication System,” which has been incorporated herein by referencein its entirety.

Once tracking has been achieved, the step of detecting the delimiter isin part accomplished by continuing to accumulate N samples (where N isthe acquisition ramp integral length, i.e., the number of samplesaccumulated in a ramp) of the repeating acquisition code to producefurther ramp values. In the embodiment where the delimiter is a flippedversion of the acquisition code sequence, the further ramp valuecorresponding to the delimiter (i.e., corresponding to the accumulatedsamples of the delimiter frames) will have a polarity opposite to rampscorresponding to the (unflipped) acquisition code sequence. Thus, thedelimiter can be detected by determining whether a delimiter thresholdhas been satisfied based on the further ramp value(s). For example,assuming a ramp value corresponding to the delimiter frames is anegative value, then the delimiter threshold can be satisfied if a rampvalue (RV) is less than a predetermined negative delimiter thresholdvalue (e.g., RV<C). More preferably, the delimiter threshold issatisfied if a ramp value is opposite in polarity from a previous rampvalue, and at least a specific fraction (e.g., ⅝) or percentage (e.g.,75%) of the previous ramp value, e.g., RVn>(RV n−1)·(0.75)·(−1).Alternatively, the delimiter can be detected by determining whether aramp value is opposite in polarity from a stored back ramp value (BRV),and at least a specific fraction (e.g., ⅝) or percentage (e.g., 75%) ofthe stored BRV, e.g., RV>(BRV)·(0.75)·(−1). The stored BRV, can be, forexample, the smallest BRV determined, as discussed above. In each ofthese embodiments, the delimiter is detected if the delimiter thresholdis satisfied. One of ordinary skill in the art will appreciate thatother delimiter threshold determinations can be used to determinewhether the delimiter has been detected, while still being within thespirit and scope of the present invention.

K. Ratchet codes

As discussed above, if the integration length is the same as the lengthof the acquisition short code, then the length of the delimiter is thesame as the length of the acquisition short code sequence (which isequal to the number of ramp builders in the receiver). In this situationthere is no need for a ratchet code of the present invention, because,after frame alignment and short code synchronization there is noambiguity relating to the boundaries of the delimeter and data payloadthat results from the integration length being greater than the numberof ramp builders. However, because the S/N ratio is typically notacceptable without the use of integration lengths longer than the lengthof the acquisition short code, the aforementioned “no ambiguity”situation is unlikely. Stated another way, to increase signal to noiseratio to an acceptable level, integration gain is typically necessary,and N is typically greater than the number of ramp builders (in theimpulse radio receiver) available for first and second stageacquisition.

Continuing with the above example, where a modulo-4 acquisition shortcode sequence is used, it is likely that samples from a plurality of(e.g., 4, 8, 16 or 32, etc.) received acquisition short codes will beaccumulated (for integration gain) to generate the above discussed rampvalues (e.g., RV0, RV1, RV2 and RV3). Again, this is done to improve S/Nratio. For example, if a receiver accumulates samples from four (4)consecutive modulo-4 acquisition short code sequences to generate eachramp value (e.g., RV0, RV1, RV2 and RV3), then each ramp value willinclude sixteen (16) amplitude samples (i.e., 1 sample/frame·4frames/short code sequence·4 short code sequences=16 samples). In otherwords, the acquisition ramp interval length in this example is sixteen(i.e., N=16). As mentioned above, the length of the delimiter ispreferably the same as the acquisition ramp integral length N (i.e., thenumber of samples accumulated in each ramp). Accordingly, in thisexample, the length of the delimiter is 16 frames. The contents of thedelimiter for this example will be explained below following adiscussion of ratchet codes. As noted above, the term “ramp intervallength” is equivalent to the term “integration length.”

Referring back to FIG. 12B, ratchet code sequence 1205 (or sequences) islocated within header 1203 of packet 1202, between the last shortacquisition code sequence 1204 and delimiter 1206. As mentioned above inthe discussion relating to FIG. 12B, ratchet code(s) (e.g., 1205) is/areused to enable a receiver to resolve the ambiguity resulting from theintegration length being greater than the number of ramp builders.Stated another way, ratchet code(s) (e.g., 1205) is/are used to enable areceiver to resolve the ambiguity relating to the start of delimiter1206 in those situations when samples are integrated to improve a signalto noise ratio. The extent of the ambiguity is equal to the integrationlength divided by the number of ramp builders (in the receiver) that areused for acquisition (the resulting ambiguity can also be stated interms of bits of ambiguity, as will be discussed below). For example, ifa receiver uses four (4) ramp builders during signal acquisition, andthe integration length is sixteen (16), then the receiver must resolve afour (4) way ambiguity (i.e., 16/4=4). This ambiguity can be explainedby way of the following example discussed with reference to FIG. 19.

FIG. 19 shows an exemplary packet 1202 that includes forty (40)acquisition short code sequences 1204, 0 through 39 (where each shortcode sequence 1204 is four frames in length), followed by delimeter 1206(which is 16 frames in length), followed by data payload 1208. In thisexample it is assumed that each accumulated ramp value (e.g., RV0, RV1,RV2, RV3) includes 16 samples (i.e., acquisition ramp interval lengthN=16) of four consecutive short code sequences. It is also assumed, forexample, that a receiver was able to achieve code and framesynchronization, and acceptable tracking, using the first thirty three(33) short code sequences 0 . . . 32 (i.e., using the first 132 framesof packet 1202, 4·33=132). Accordingly, at this point (i.e., beginningwith the 133rd frame of packet 1202) assume that the receiver begins togenerate accumulated ramp values, each including 16 samples, to attemptto detect delimeter 1206. Because short acquisition code and framesynchronization have been achieved, the receiver knows where eachimpulse is within each frame of the short acquisition code sequences1204. As discussed above, the receiver attempts to detect delimeter 1206by determining whether a delimiter threshold has been satisfied. Variousways of determining whether the delimeter threshold has been satisfiedhave been discussed above.

Still referring to FIG. 19, assuming the use of modulo-4 short codes,and a ramp integral length N=16 (and thus a delimiter length=16), it ispossible that a ramp builder of the receiver generates accumulated rampvalues corresponding to the portions of packet 1202 designated Ramp A,Ramp B and Ramp C (referred, to respectively as ramp value A (RVA), rampvalue B (RVB) and ramp value C (RVC)). As shown, RVB (corresponding toRamp B) includes samples of three (3) short acquisition code sequences1204 (short code sequences 37, 38 and 39) and one fourth (¼) ofdelimeter 1206. More specifically, RVB includes twelve (12) frames ofshort acquisition code 1204 (i.e., 3 code sequences·4 frames/codesequence=12 frames) and four frames (4) of delimeter 1206. Also shown,RVC includes samples of three fourths (¾) of delimeter 1206 and a smallportion of data payload 1208. If ramp values are produced in thismanner, the receiver will not detect delimiter 1206 because neither RVBnor RVC will satisfy the delimeter threshold. Thus, the receiver willcontinue to search for delimeter 1206 in data payload 1208, where thedelimeter certainly will not be found. Accordingly, the receiver may notbe able to demodulate the data of data payload 1208.

If the receiver generates accumulated ramp values corresponding toportions of packet 1202 designated Ramp D, Ramp E and Ramp F, thereceiver will again not detect delimeter 1206, for reasons similar tothose just discussed above. Similarly, if the receiver generatesaccumulated ramp values corresponding to portions of packet 1204designated Ramp G, Ramp H and Ramp I, the receiver will not detectdelimeter 1206.

Each of the above scenario occurs because of ambiguities relating to thestart of delimiter 1206, resulting from the integration length beinglonger than the length of the acquisition short code. That is, the abovescenario occurs because the receiver does not know where the series ofrepeating short code sequences ends and the delimiter begins. Becausethe integration length is 16, and the acquisition short code length is4, the delimiter can start at one of four different possible positionsover an integration period of 16 frames. The only way the receiver willdetect delimeter 1206 is if it generates an accumulated ramp valuecorresponding to the portion of packet 1202 designated Ramp K. This isbecause such an accumulated ramp value (i.e., RVK) will consist solelyof samples of delimeter 1206, and the delimeter threshold should besatisfied.

Accordingly, as can be appreciated from the above example, a reasonthere is an ambiguity relating to the start of delimiter 1206 is becauseall of the repeating acquisition short code sequences 1204 look exactlyalike to the receiver. Thus, the receiver does not know when theacquisition short code sequences 1204 end, and delimeter 1206 begins.This presents no ambiguity if the acquisition ramp integral length N isequal to the length of acquisition short code 1204 (as was the casedescribed above in connection with FIGS. 17 and 18), and thus equal tothe length of delimeter 1206. However, as shown in the example discussedin connection with FIG. 19, this is a problem when the acquisition rampintegral length N, and the length of delimiter 1206, are integermultiples of the length of acquisition short code 1204. More generally,the issue is that the receiver includes insufficient hardware to resolveall possible ambiguities in parallel.

As shall now be explained, the ratchet codes of the present inventionenable the receiver to efficiently detect delimiter 1206, and thus todetect the start of data payload 1208. As will be appreciated, theratchet codes becomes much more important as the integration length isincreased as compared to the number of ramp builders. For example, ifthe integration length in the example of FIG. 19 is 32 (i.e., N=32),then there are 8 ambiguities (i.e., 32/4=8) as to where the delimiterbegins. This eight way ambiguity can be resolved, for example, with only4 ramp builders using the ratchet codes of the present invention, asshall be explained below.

Another way of discussing the ambiguities that must be resolved are interms of bits of ambiguities. The total number of bits of ambiguity thatmust be resolved during first and second stage acquisition, together,equals:

log 2(integration length).

For example, if the integration length is 4, then only 2 bits ofambiguity must be resolved, i.e., log 2(4)=2. For another example, ifthe integration length is 16, then 4 bits of ambiguity must be resolved,i.e., log 2(16)=4. If the integration length is 256, then 8 bits ofambiguity must be resolved, i.e., log 2(256)=8.

The essence of acquisition is resolving ambiguities that exist withregards to frame alignment and code alignment, and in the presentinvention, integration length. During first stage acquisition, asexplained above, the number of integrations (which is the same as thenumber of ramps that are built by each ramp builder) that are requiredto resolve the ambiguity relating to frame alignment (worst case)equals: (frame length/frame step size) rounded up to next integer.

For example, if frame length is 100 ns, and the step size is 3 ns, thenapproximately 34 integrations (worst case) are required to resolve theambiguity relating to frame alignment, i.e., rounded up (100/3)=34. Asexplained above, during first stage acquisition, code alignment orsynchronization is achieved to the extent of the short acquisition code.Stated another way, the number of bits of ambiguity resolved duringfirst stage acquisition equals:

log 2(short acquisition code length).

For example, if the short acquisition code length=4, then 2 bits ofambiguity are resolved during first stage acquisition. In anotherexample, if the short code length is 16, then 4 bits of ambiguity areresolved during first stage acquisition.

The number of bits of ambiguity that must be resolved during secondstage acquisition is equal to the number of bits that were not resolvedduring first stage acquisition. More specifically, the number of bitsthat must be resolved during second stage acquisition equals:

log 2(integration length)−log 2(short acquisition code length).

For example, if the integration length is 4 (i.e., N=4), and the shortacquisition code length is 4, then 0 bits of ambiguity must be resolvedduring second stage acquisition (i.e., log 2(4)−log 2(4)=2−2=0).However, if the integration length is 16, and the short acquisition codelength is 4, then 2 bits of ambiguity must be resolved during secondstage acquisition (i.e., log 2(16)−log 2(4)=4−2=2). For another example,if the integration length is 256, and the short acquisition code is 4,then 6 bits of ambiguity must be resolved during second stageacquisition (i.e., log 2(256)−log 2(4)=8−2=6). As will be explained indetail below, the ratchet codes of the present invention are used toresolve the number of bits of ambiguity that must be resolved duringsecond stage acquisition.

In the example discussed above in connection with FIGS. 13, 17 and 18(which were used to explain parallel steps 1602 a-d and 1604 a-d of FIG.16), four accumulated ramp values (RV0, RV1, RV2 and RV3) wererepeatedly generated, in parallel, to achieve frame and codesynchronization. A specific amount of receiver hardware (i.e., rampbuilders) is required, as will be explained below, to generate theseramp values. For example, assume four correlators and four ramp buildersare required to sample each frame (e.g., of modulo-4 acquisition shortcode 1204) at four different times (or positions), and integrate thesamples, as shown in FIGS. 17 and 18. It is preferable that the receivercan resolve the ambiguity relating to the beginning of delimeter 1206without requiring additional hardware. Phrased an alternative way, it ispreferable that the receiver can resolve the ambiguities relating to theintegration length being greater than the number of ramp builders,without requiring additional hardware. Stated still another way, it ispreferable that the receiver can resolve the number of ambiguities thatmust be resolved during second stage acquisition, without additionalhardware. The present invention achieves this using ratchet codes, asshall now be explained.

Whether a ratchet code is used, the number of different ratchet codes,the contents of each ratchet code, and how many of each ratchet code arein a packet header, are all defined by the packet protocol which isknown by both the transmitting and receiving impulse radios.

The number of unique ratchet codes required to resolve the abovediscussed ambiguities that must be resolved during second stageacquisition is a function of the integration length, the shortacquisition code length, and the number of ramp builders available(i.e., used) for signal acquisition. More specifically, the number ofunique ratchet codes required to resolve the ambiguities that must beresolved during second stage acquisition is equal to:

${rounded}\mspace{14mu} {up}\mspace{14mu} {to}\mspace{14mu} {next}\mspace{14mu} {integer}{\left\{ \frac{\begin{matrix}{{\log_{2}\left( {\text{int}.{\_ length}} \right)} -} \\{\log_{2}\left( {{{short\_ aqu}.{\_ code}}{\_ length}} \right)}\end{matrix}}{\log_{2}\left( {\# {\_ of}{\_ available}{\_ ramp}{\_ builders}} \right)} \right\}.}$

The numerator of the above equation is equal to the number of bits thatmust be resolved during second stage acquisition. The denominator is log2 of the number of ramp builders available (i.e., used) for signalacquisition during first stage acquisition, and during second stageacquisition (this does not include any back ramp builders). Preferably,as mentioned above, the same ramp builders are used during first stageacquisition as during second stage acquisition. Also, the length of eachratchet code is greater than the length of the short acquisition code.

If, for example, two unique ratchet codes are required, then the lengthof the second ratchet code is greater than the length of the firstratchet code, which is greater than the length of the short acquisitioncode. The length of the longest ratchet code will be equal to theintegration length. More generally, each additional unique ratchet codeis longer than the previous unique ratchet code.

The number of each unique ratchet code sequences that must be includedin a packet transmitted from an impulse radio transmitter to an impulseradio receiver is preferably as follows:

-   -   1) one sequence of the longest unique ratchet code; and    -   2) two sequences of each of the other (i.e., shorter) unique        ratchet code(s).

Thus, if only one unique ratchet code is required, then only one ratchetcode sequence must be included in a packet transmitted from thetransmitter to receiver. However, if two unique ratchet codes arerequired, then two of the first (i.e., shorter) ratchet code sequencesare included in a packet, and one of the second (i.e., longer) ratchetcode sequences are included in the packet. Once again, this is definedby the packet protocol that is known by impulse radios in communicationswith one another. The sending of more than the above specified number ofratchet codes is within the spirit and scope of the present invention.

The length of the longest ratchet code is preferably equal to theintegration length. The length of the first ratchet code can be as longas the short acquisition code length multiplied by the number ofavailable ramp builders. The length of each additional ratchet code (ifnecessary) can be as long as the previous ratchet code multiplied by thenumber of available ramp builders, keeping in mind, that the length ofthe longest ratchet code is preferably equal to the integration length.

FIG. 20 shall be used to explain how a ratchet code of the presentinvention can be used to resolve the ambiguities that occur when areceiver uses four (4) available ramp builders during signalacquisition, the short acquisition code is length four (4), and theintegration length is sixteen (16). Thus, in this example, the number ofbits of ambiguity resolved during first stage acquisition is 2 (i.e.,log 2(4)=2). The number of ambiguities that must be resolved duringsecond stage acquisition is 2 (i.e., log 2(16)−log 2(4)=4−2=2). Thenumber of unique ratchet codes required to resolve the ambiguities thatmust be resolved during second stage acquisition is equal to 1, i.e.,rounded up to next integer {(log 2(16)−log 2(4))/log2(4)}={(4−2)/2}={2/2}=1. One sequence of the one unique ratchet codeshould be included in a packet. The length of the ratchet code ispreferably 16. FIG. 20 is also used to explain the steps of FIG. 21.

FIG. 21 illustrates additional details of step 1508 of method 1500,according to an embodiment of the present invention. The steps discussedin connection with FIG. 21 correspond to a method of detecting thebeginning of a data payload, after frame alignment was achieved based ona modulo-4 acquisition code and repeating modulo-4 acquisition codesequences of the received signal, and code synchronization was achievedto the extent of the short acquisition code. Step 1508 occurs after step1504 (explained in above additional detail in connection with FIG. 16),and after signal tracking at step 1506. Thus, the receiver can predictthe various possible beginning boundaries of a ratchet code, since thereceiver knows where frame boundaries are at this point. Based on thedescription herein, one of ordinary skill in the art will appreciate howthis method can be modified to achieve frame alignment based on shortacquisition codes of other lengths (e.g., a modulo-8 acquisition code, amodulo 16-acquisition code, etc.). Based on the description herein, oneof ordinary skill in the art will also appreciate how ratchet codes ofthe present invention can be used to resolve additional ambiguities thatwould result from increased integration lengths.

As shown in FIG. 21, at a step 2102 a, a plurality of frames (i.e., Nframes, where N is the integration length) of the received impulsesignal packet are sampled based on a ratchet code (which is known orderived by the impulse radio receiver) and a code boundary CB0 (i.e.,CB=0), to thereby produce a plurality of samples corresponding to thecode boundary. For example, if the ratchet code has a length of sixteen(16) frames, then sixteen frames of the received signal packet aresampled based on the known or derived ratchet code, and a guess that theratchet code begins at the short code boundary designated CB0 in FIG.20. The sixteen frames sampled at step 2102 a are designated RatchetRamp 0 in FIG. 20.

Concurrently (but beginning slightly later in time, e.g., one codelength later in time), at a step 2102 b, the plurality of frames of thereceived impulse signal packet are sampled based on the ratchet code anda different code boundary CB1 (i.e., CB=1), to thereby produce aplurality of samples corresponding to the code boundary CB1. The sixteenframes sampled at step 2102 b are designated Ratchet Ramp 1 in FIG. 20.

Concurrently (but beginning slightly later in time), at a step 2102 c, aplurality of frames of the received impulse signal packet are sampledbased on the ratchet code and a different code boundary CB2 (i.e.,CB=2), to thereby produce a plurality of samples corresponding to thecode boundary CB2. The sixteen frames sampled at step 2102 c aredesignated Ratchet Ramp 2 in FIG. 20.

Concurrently (but beginning slightly later in time), at a step 2102 d, aplurality of frames of the received impulse signal packet are sampledbased on the ratchet code and a different code boundary CB3 (i.e.,CB=3), to thereby produce a plurality of samples corresponding to thecode boundary CB3. The sixteen frames sampled at step 2102 d aredesignated Ratchet Ramp 3 in FIG. 20.

At next parallel steps 2104 a, 2104 b, 2104 c and 2104 d, the pluralityof samples produced at steps 2102 a, 2102 b, 2102 c and 2102 d, are,separately accumulated to produce respective ramp values (also referredto as accumulated values). More specifically, at step 2104 a, theplurality of samples corresponding to the code boundary CB0 areaccumulated (e.g., by a ramp builder) to produce a ratchet ramp valueRRV0. Concurrently, at step 2104 b, the plurality of samplescorresponding to the code boundary CB1 are accumulated (e.g., by aseparate ramp builder) to produce a ratchet ramp value RRV1.Concurrently, at step 2104 c, the plurality of samples corresponding tocode boundary CB2 are accumulated (e.g., by another ramp builder) toproduce a ratchet ramp value RRV2. Concurrently, at step 2104 d, theplurality of samples corresponding to the code boundary CB3 areaccumulated (e.g., by still another ramp builder) to produce a ratchetramp value RRV3.

At a next step 2106, there is a determination of whether a ratchetthreshold has been satisfied. This determination is based on one or moreof the ratchet ramp values RRV0, RRV1, RRV2 and RRV3. Various methodsfor performing this threshold determination, according to the presentinvention, are discussed below. For example, the ratchet threshold canbe satisfied if a ratchet ramp value is at least a specific fraction(e.g., ⅝) or percentage (e.g., 75%) of a previous ramp value (e.g., aprevious highest ramp value). Alternatively, the ratchet code thresholdcan be satisfied if a ratchet ramp value is at least a specific fraction(e.g., ⅝) or percentage (e.g., 75%) of the stored back ramp value (BRV),e.g., RRV>(BRV)·(0.75). Preferably, the ratchet code threshold issatisfied when RRV>(BRV)·(Back Ramp Factor), where BRV is the smallestback ramp determined during tracking. Possible values for the back rampfactor are, for example, 1, ⅞, ¾, and ⅝. In each of these embodiments,the ratchet code is detected if the ratchet threshold is satisfied. Oneof ordinary skill in the art will appreciate that other ratchetthreshold determinations can be used to determine whether a ratchet codesequence has been detected.

The threshold may not be satisfied, for example, if the receiver issampling in accordance with the ratchet code when the actual portion ofthe packet header being sampled still includes short acquisition codesequences. Accordingly, if the answer to step 2106 is NO, then flowreturns to parallel steps 2102 a, 2102 b, 2102 c and 2102 d so thatadditional frames of the packet header can be sampled in search of aratchet code sequence. If flow indefinitely returns to parallel steps2102 a, 2102 b, 2102 c and 2102 d, then frame alignment and/or shortcode synchronization has most likely been lost. Thus, preferably, ifflow returns to parallel steps 2102 a, 2102 b, 2102 c and 2102 d apredetermined number of times, then flow jumps back to step 1502 andfirst stage acquisition starts over again.

The threshold will be satisfied (i.e., the answer to step 2106 is YES)when one of the accumulated ratchet ramp values (i.e., RRV3 in thisexample, as shown in FIG. 20) consists solely of samples of ratchet codesequence 1205. For this example it is assumed that the packet protocolonly defines one unique ratchet code. A more general flow diagramapplicable to various packet protocols (that may define more than oneunique ratchet code) shall be discussed below, with reference to FIG.22.

If the answer to step 2106 is YES (i.e., if a ratchet code sequence isdetected), then, in this example, flow goes to step 2110, as shown. Atthis point, in this example, the ambiguity resulting from theintegration length being greater than the number of ramp builders hasbeen resolved. Accordingly, the receiver now samples the received signalin an attempt to detect the delimeter. The delimeter is defined by thepacket protocol. According to a preferred embodiment of the presentinvention, the delimeter is a flipped version of the longest ratchetcode of the packet protocol. Thus, at step 2110, a plurality of framesare samples based on the longest ratchet code of the protocol. In thisexample there is only one rachet code, which was 16 frames in length.

Next, at a step 2112, there is a determination of whether the delimeterthreshold has been satisfied. Various ways of performing this thresholddetermination have been discussed in detail above.

If the answer to step 2112 is NO (i.e., if the delimeter was notdetected), then the receiver has probably lost frame alignment, andthus, flow jumps back to step 1502 and first stage acquisition startsover again. If the answer to step 2112 is YES, then flow goes to step1510, where data payload symbols are detected. At this point symboltiming has been determined because the impulse radio knows preciselywhen the first frame of the data payload begins and the receiver knowsthe long code used to encode the symbols in the data payload (e.g.,based on the packet protocol).

FIG. 22 shall now be used to illustrate the details of step 1508 ofmethod 1500, in more general terms, according to an embodiment of thepresent invention. At a step 2201, a value X is set to zero (X is usedto track the number of times flow returns to step 2102 due to theratchet threshold not being satisfied). At step 2102, the receivedsignal is sampled at a plurality of times in accordance with the ratchetcode and a code boundary, to produce sequences of samples correspondingto a code boundary. Next at step 2104, the sequence of samples arecombined to produce a ratchet ramp value corresponding to the codeboundary. As shown, steps 2102 and 2104 are concurrently performed foreach of n different code boundaries, thereby producing n ratchet rampvalues each corresponding to one of the different code boundaries.

At step 2106 there is a determination of whether a ratchet threshold issatisfied based on the n ratchet ramp values. This has been explained indetail above.

If the answer to step 2106 is NO, then X=X+1 (at step 2203) and there isa determination of whether X<MaxX (e.g., 4), which is a predeterminedvalue (at step 2205). If the answer to step 2205 is YES, then flowreturns to step 2102 so that the ratchet code can be searched for again.If the answer to step 2205 is NO, then it is assume that the receiverhas lost frame alignment, and thus, flow jumps back to step 1502 andfirst stage acquisition starts over again.

If the answer to step 2106 is YES, then synchronization has beenachieved to the length of the ratchet code, and flow goes to step 2208,as shown. In other words, if the answer to step 2106 is YES, thenambiguity resulting from the integration length being greater than thenumber of ramp builders has been resolved to the extent of the length ofthe ratchet code.

At step 2208, there is a determination of whether there are additionalratchet codes for which to search. This determination is based on thepacket protocol which is known by both the transmitting and receivingimpulse radios. The packet protocol, as discussed above, preferablydefines each ratchet code, defines how many different ratchet codes arewithin each packet, and defines how many of each different ratchet codeare within each packet. If the answer to step 2208 is YES, then at step2209 the ratchet code is set to the next ratchet code (i.e., ratchetcode=next ratchet code). For example, the answer to step 2208 will beYES at least once, when the integration length is longer than the firstratchet code. Flow then returns to step 2102, and steps 2102 through2106 are repeated using the next ratchet code. It is noted that thefirst time step 2102 is performed, the code boundary is defined by theshort acquisition code. If step 2102 is performed in search ofadditional ratchet codes (i.e., if the answer to step 2208 is YES), thenthe code boundary will be defined by the previous ratchet code (e.g., ifthe second ratchet code is being searched for, the code boundaries aredefined by the first rachet code), in a similar manner as when the codeboundaries are defined by the short acquisition code, as discussedabove.

As mentioned above, a packet protocol may specify that a packet includesmore than one different ratchet code sequence because it may not bepossible to resolve the ambiguity, resulting from the integration lengthbeing greater than the number of ramp builders, using just one ratchetcode sequence. For example, assume a receiver uses eight (8) rampbuilders during signal acquisition, the short acquisition code is lengthsixteen (16), and the integration length is two hundred and fifty six(256). Thus, in this example, the number of bits of ambiguity resolvedduring first stage acquisition is 4 (i.e., log 2(16)=4). The number ofambiguities that must be resolved during second stage acquisition is 4(i.e., log 2(256)−log 2(16)=8−4=4). The number of unique ratchet codesrequired to resolve the ambiguities that must be resolved during secondstage acquisition is equal to 2, i.e., rounded up to next integer {(log2(256)−log 2(32))/log 2(8)}=rounded up to next integer {(8−4)/3}=roundedup to next integer {4/3}=2. Two sequences of a shorter one of the uniqueratchet codes should be included in a packet, and one sequence of thelonger ratchet code should be included. The length of the shorterratchet code is preferably 128 (i.e., 16·8=128). The length of thelonger ratchet code is preferably 256 (the length can be up to 1024because 128·8=1024, but the length of the longest ratchet code ispreferably equal to the integration length).

Returning to the discussion of FIG. 22, if the answer to step 2208 is NO(indicating that the ambiguity resulting from the integration lengthbeing greater than the number of ramp builders has been resolved), thenflow goes to step 2110, and the receiver samples the received signal inan attempt to detect the delimeter (defined by the packet protocol). Asmentioned above, according to a preferred embodiment of the presentinvention, the delimeter is a flipped version of the longest ratchetcode of a packet protocol. Accordingly, at step 2110, a plurality offrames are samples based on the longest ratchet code of the protocol.

Next, at step 2112, there is a determination of whether the delimeterthreshold has been satisfied. Various ways of performing this thresholddetermination have been discussed in detail above.

If the answer to step 2112 is NO (i.e., if the delimeter was notdetected), then the receiver has probably lost frame alignment, andthus, flow jumps back to step 1502 and first stage acquisition startsover again. In another embodiment, the answer to step 2112 must be NO apredetermined number of times (e.g., 3) before flow returns to step1502. If the answer to step 2112 is YES, then flow goes to step 1510,where data payload symbols are detected. At this point symbol timing hasbeen determined because the impulse radio knows precisely when the firstframe of the data payload begins and the receiver knows the long codeused to encode the symbols in the data payload (based on the packetprotocol). As discussed below, in an embodiment of the presentinvention, command information is included between the delimeter anddata payload.

L. Generating Ratchet Codes

FIG. 23 illustrates how ratchet codes can be generated based on a shortacquisition code. The left most column 2302 of table 2300 represents ashort acquisition code stored, for example, in an acquisition rootregister of an impulse radio transceiver. The additional columns showhow longer ratchet codes can be derived by “hopping” or “skipping”positions of the short acquisition code. These additional columns neednot be stored in the impulse radio receiver. Rather, an acquisitionsequence generator of the impulse radio receiver can generate theportion of a code, shown in the additional columns, based on the shortacquisition code stored in the register (i.e., column 2302), as will beexplained below.

During first stage acquisition, an acquisition sequence generator of theimpulse radio transceiver can simply cycle through the short acquisitioncode (e.g., stored in the acquisition root register, and shown at theleft most column 2302 of table 2300) from top to bottom going throughthe length four code. To generate longer codes (e.g., ratchet codes)based on the modulo-4 short acquisition code, the logic (e.g., of theacquisition sequence generator) goes through the left column once butinstead of starting over at 0 (i.e., the zeroth code position) after theend of the first column 2302, the logic “hops” or “skips” one codeposition. This causes the sequence to hop over code position 0 and startat code position 1. The second column from the left shows the sequenceafter one hop. Thus, if the short code is P0, P1, P2, P3 (as shown inthe left most column 2302), then a length 8 ratchet code, generated inaccordance with an embodiment of the present invention, is P0, P1, P2,P3, P1, P2, P3, P0 (represented by the first two columns of table 2300).

If additional longer ratchet codes are required for first stageacquisition, then the process repeats each time the end of a column isreached until the desired code length is obtained. For example, a length16 ratchet code is obtained by cycling through the short acquisitioncode four times, each time hopping another one position. This isrepresented in the first four columns of the table 2300. According thelength 16 ratchet code is P0, P1, P2, P3, P1, P2, P3, P0, P2, P3, P0,P1, P3, P0, P1, P2.

Codes up to length 16 can be generated using a length 4 short code andsingle hops. For codes longer than 16, two hops are required to break upthe sequence. For example, to generate a length 32 code, the logicstarts at the upper left corner of table 2300, and single hops at theend of each column until the end of the fourth column. At that point,two positions are hopped to produce the fifth column. Then one positionis hopped to produce the sixth, seventh and eighth columns. For example,a length 32 ratchet code can be obtained using all eight columns shownin table 2300, wherein the ratchet code is P0, P1, P2, P3, P1, P2, P3,P0, P2, P3, P0, P1, P3, P0, P1, P2, P1, P2, P0, P2, P3, P0, P1, P3, P0,P1, P2, P0, P1, P2, P3. To generate even longer codes double hops areperformed at each length 32 code boundary. As stated above, the secondthrough eighth columns need not be stored in the impulse radio receiver.Rather, the acquisition sequence generator of the impulse radio receivercan generate the portions of a code shown in the additional columns(i.e., the second through eighth columns) based on the short acquisitioncode stored in the register (i.e., column 2302).

When discussing possible positions within a frame (where an impulse canbe located, e.g., due to a code), the nomenclature Pn (e.g., P0, P1, P2,P3) has been used to represent the different possible (e.g., four)positions. It is noted, however, that position P0, need not be aposition that is earlier in a frame (i.e., closer to the beginning frameboundary) than positions P1, P2 and/or P3. Rather, references toposition P0, P1, P2, P3 simply refer to four different possiblepositions within a frame. The order of the positions as shown in thefigures (e.g., FIGS. 12A, 13, 14, 17 and 18) was to simplify thediscussion.

According to an embodiment of the preset invention, the packet protocolis such that every ratchet code can be derived from a short acquisitioncode that is stored in a register of an impulse radio. For example, aregister can include the contents of first column 2302 of table 2300.The ratchet codes can be derived from the contents of first column 2302as described above in the discussion of FIG. 23. Similarly, a registercan include the contents of first column 2802 of table 2800. The ratchetcodes can be derived from the contents of first column 2802 as describedbelow in the discussion of FIG. 28. Thus, a packet protocol of thepresent invention enables a minimum amount of data (e.g., the shortacquisition code) to be stored in a register that is used to derivecodes for signal acquisition.

M. Exemplary Impulse Radio Receiver Subsystem

FIG. 24 shows an exemplary portion of an impulse radio receiver 2402(simply referred to as receiver 2402, or receiver subsystem 2402) of thepresent invention. This exemplary receiver 2402 can be used to implementthe methods of the present invention that have been explained above.Receiver 2402 can be implemented using receiver 600 of FIGS. 6 and 7,with elements not explicitly shown in FIGS. 6 and 7, for example,implemented in baseband processor 620.

1. Operation During First Stage Acquisition

Referring to FIG. 24, exemplary receiver 2402 includes four samplingcorrelators 726 a, 726 b, 726 c, 726 d. During first stage acquisition:correlator 726 a samples a zeroth impulse position P0 in accordance witha sampling control signal 736 a; correlator 726 b samples a firstimpulse position P1 in accordance with a sampling control signal 736 b;correlator 726 c samples a second impulse position P2 in accordance witha sampling control signal 736 c; and correlator 726 d samples a thirdimpulse position P3 in accordance with a sampling control signal 736 d.First stage acquisition has previously been described in connection withstep 1502 of FIG. 15, and also in connection with additional figures,such as FIGS. 16, 17 and 18. Accordingly, earlier discussed figures willbe referred to below where applicable.

A resulting Sample/Hold (S/H) signal 728 a (also referred to as S0),representing correlation results from correlator 726 a, is provided tofour multiplexers, including Mux0, Mux1, Mux2 and Mux3. Similarly,resulting Sample/Hold (S/H) signal 728 b (also referred to as S1),representing correlation results from correlator 726 b, is provided toMux0, Mux1, Mux2 and Mux3. Resulting Sample/Hold (S/H) signal 728 c(also referred to as S2), representing correlation results fromcorrelator 726 c, is provided to Mux0, Mux1, Mux2 and Mux3. ResultingSample/Hold (S/H) signal 728 d (also referred to as S3), representingcorrelation results from correlator 726 d, is also provided to Mux0,Mux1, Mux2 and Mux3.

In accordance with specific embodiments of the present inventiondiscussed above, correlators 726 a, 726 b, 726 c and 726 d sample eachof the possible impulse positions during a frame period (four possiblepositions in this example) in an attempt to sample impulses of repeatingshort acquisition codes of a packet header. The outputs S0, S1, S2 andS3 of the correlators are provided to multiplexers Mux0, Mux1, Mux2 andMux3. Mux0 selects one of outputs S0, S1, S2 and S3 in accordance withthe short acquisition code (P0, P1, P2, P3) and a zero code offset(CO0), represented by the first column of table 2404. Sample timelines1702 a and 1802 a are useful for understanding the operation of Mux0.Mux1 selects one of outputs S0, S1, S2 and S3 in accordance with theshort acquisition code (P0, P1, P2, P3) and a code offset=1 (CO1),represented by the second column of table 2404. Sample timelines 1702 band 1802 b are useful for understanding the operation of Mux1. Mux2selects one of outputs S0, S1, S2 and S3 in accordance with the shortacquisition code (P0, P1, P2, P3) and a code offset=2 (CO2), representedby the third column of table 2404. Sample timelines 1702 c and 1802 care useful for understanding the operation of Mux2. Mux3 selects one ofoutputs S0, S1, S2 and S3 in accordance with the short acquisition code(P0, P1, P2, P3) and a code offset=3 (CO3), represented by the fourthcolumn of table 2404. Sample timelines 1702 d and 1802 d are useful forunderstanding the operation of Mux3.

Accordingly, correlators 726 a, 726 b, 726 c and 726 d perform steps1602 a, 1602 b, 1602 c and 1602 d, discussed above in connection withFIG. 16, and also explained in connection with FIGS. 17 and 18.

It is noted that table 2404 need not be stored in a memory of receiver2402. Rather, only a first column of table 2404 is stored in a memory orregister (e.g., a short code register) of receiver 2402, and thecontents of the other columns (i.e., the short acquisition code withcode offsets CO1, CO2 and CO3) are generated based on the contents ofthe register (e.g., by shifting the contents of the register, or bybeginning at a different point in a rotating register).

Mux0 outputs a signal 2406 a, which includes a plurality of samplescorresponding to the code offset CO0 and a frame offset (FO). Thesesamples corresponding to the code offset CO0 and the frame offset (FO)(i.e., signal 2406 a) are provided to a ramp builder R0 (also referredto as an accumulator R0), which accumulates N samples (where N is theintegration length), and outputs a ramp value RV0.

Mux1 outputs a signal 2406 b, which includes a plurality of samplescorresponding to the code offset CO1 and the same frame offset (FO).These samples corresponding to the code offset CO1 and the frame offset(FO) (i.e., signal 2406 b) are provided to a ramp builder R1 (alsoreferred to as an accumulator R1), which accumulates N samples (where Nis the integration length), and outputs a ramp value RV1.

Mux2 outputs a signal 2406 c, which includes a plurality of samplescorresponding to the code offset CO2 and the same frame offset (FO).These samples corresponding to the code offset CO2 and the frame offset(FO) (i.e., signal 2406 c) are provided to a ramp builder R2 (alsoreferred to as an accumulator R2), which accumulates N samples (where Nis the integration length), and outputs a ramp value RV2.

Mux3 outputs a signal 2406 d, which includes a plurality of samplescorresponding to the code offset CO3 and the same frame offset (FO).These samples corresponding to the code offset CO1 and the frame offset(FO) (i.e., signal 2406 d) are provided to a ramp builder R3 (alsoreferred to as an accumulator R3), which accumulates N samples (where Nis the integration length), and outputs a ramp value RV3.

Accordingly multiplexers Mux0, Mux1, Mux2 and Mux3, and ramp buildersR0, R1, R2 and R3 perform steps 1604 a, 1604 b, 1604 c and 1604 d,discussed above in connection with FIG. 16, and also in connection withFIGS. 17 and 18.

Ramp values RV0, RV1, RV2 and RV3 are provided to a threshold detector2408, which determines whether a threshold (e.g., a first stageacquisition threshold) has been satisfied. Thus, threshold detector 2408performs step 1606 discussed above in connection with FIG. 16. Thisdetermination is based on one or more of ramp values RV0, RV1, RV2 andRV3. Various threshold determinations, according to the presentinvention, have been discussed in detail above.

If the threshold has been satisfied, then receiver 2402 determines thatit has identified the correct frame offset and code offset, and thus,the impulse radio receiver has achieved frame alignment and shortacquisition code synchronization. Failure to satisfy the threshold willtypically indicate that the sample frames are not frame aligned with theframes of the received packet. Accordingly, failure to satisfy thethreshold shall be interpreted as indicating that there is anunacceptable offset between the beginning of each frame in the receivedpacket and the beginning of each frame of the sample frames. Thus, ifthe threshold has not been satisfied, then the frame offset FO isadjusted (e.g., FO=FO+3 nsec, where 3 nsec is referred to as the stepsize) so that another frame offset can be tested. After the frame offsetFO has been adjusted, each of correlators 726 a, 726 b, 726 c and 726 dattempts to sample additional frames of repeating short code sequences,in accordance with a specific code offset (e.g., respectively, CO0, CO1,CO2 and CO3) and the new frame offset FO. New ramps values RV0, RV1, RV2and RV3 are produced in the same manner discussed above. This process isrepeated, each time with a new frame offset FO, until the threshold issatisfied, thereby indicating that impulse radio receiver 2402 hasachieved satisfactory frame alignment (also referred to as framesynchronization), and also short acquisition code synchronization.

It is noted that analog to digital converters (e.g., A/Ds 772 a-d) arenot explicitly shown in FIG. 24. However, it is likely that A/Ds areplaced at the output of each correlator, so that the inputs to themultiplexers are digital signals. Alternatively, such A/Ds can be placedbetween the multiplexers and the ramp builders, or between the rampbuilders and threshold detector 2408. Additionally, certain elements ofthe receiver front end are not shown in FIG. 24.

2. Tracking and Back Ramps

After first stage acquisition, one or more of correlators 726 a, 726 b,726 c and 726 d can be used for tracking, as discussed in more detailabove (e.g., in connection with FIG. 11).

After first stage acquisition is complete and preferably after trackinghas been initiated, impulse radio receiver 2402 searches for appropriateratchet code(s), as part of second stage acquisition. Impulse radioreceiver 2402 begins this process by sampling the received signal inaccordance with the first expected ratchet code. As explained above, itis likely that receiver 2402 will sample the received signal inaccordance with the first expected ratchet code while the receivedpacket still consists of the repeating short acquisition code sequences.This occurs when receiver 2402 completes first stage acquisition usingless than all of the repeating short acquisition code sequences of thepacket (e.g., if first stage acquisition is achieved using 32 of 40repeating short acquisition code sequences). During this intermediateperiod, there is the possibility that tracking of the received signal isadversely affected because the tracking loop may operate in a partialopen loop fashion. This can reduce the signal to noise ratio of samples,and may even cause the oscillator in impulse radio receiver 2402 todrift (with respect to the oscillator of the impulse radio transmitter)to the extent that acquisition of the packet is lost.

Accordingly, in an embodiment of the present invention, ramp valuescontinue to be built based on the short acquisition code, during secondstage acquisition. Referring now to FIG. 25, in an embodiment of thepresent invention, when first stage acquisition is complete, thedetected short acquisition code and its code offset is loaded into aback register and/or a back sequence generator 2502. Samples generatedin accordance with the short acquisition code and the appropriate codeoffset (as selected by BackMux) are then used in the tracking loop,instead of samples generated in accordance with ratchet codes. Morespecifically, Back Ramp builder (BR) generates back ramp values (BRVs)according to the short acquisition code (stored in back sequencegenerator or register 2502), during second stage acquisition.

In accordance with an embodiment of the present invention, the smallestback ramp value (BRV) is stored in a memory or register (which may ormay not be part of threshold detector 2408) so that it can be used forthreshold determination(s) of second stage acquisition. Each time asmaller BRV is determined, it replaces the previously stored smallestBRV. In another embodiment of the present invention, a largest BRV isstored. In still another embodiment, an average of many BRVs isdetermined and stored. In a further embodiment of the present invention,the latest BRV generated is stored. The use of BRVs has been discussedabove, and will be described again below in the discussion of receiveroperation during second stage acquisition.

3. Operation During Second Stage Acquisition

In accordance with a preferred embodiment of the present invention, thesame hardware used for first stage acquisition is also used for secondstage acquisition. Second stage acquisition has previously beendescribed in connection with step 1508 of FIG. 15, and also inconnection with additional figures, such as FIGS. 20, 21 and 22.Accordingly, these figures are referenced below when appropriate.

Referring again to FIG. 24, during second stage acquisition, samplingcorrelator 726 a samples a plurality of frames (i.e., N frames, where Nthe integration length) of the received impulse signal packet based on aratchet code (which is known or derived by the impulse radio receiver)and a code boundary CB0 (i.e., CB=0), to thereby produce a plurality ofsamples corresponding to the code boundary. For example, if the ratchetcode has a length of sixteen (16) frames, then sixteen frames of thereceived signal packet are sampled based on the ratchet code, and aguess that the ratchet code begins at a short code boundary CB0 (e.g.,shown in FIG. 20, discussed above). For example, the sixteen framessampled by correlator 726 a are designated Ratchet Ramp 0 in FIG. 20.Accordingly, correlator 726 a is used to perform step 2102 a, discussedabove in connection with FIGS. 21 and 22. Receiver 2402 preferablyderives the ratchet code(s) based on the acquisition short code storedin a register, as explained in detail above.

Concurrently (but beginning slightly later in time), sampling correlator726 b samples a plurality of frames of the received impulse signalpacket based on the ratchet code and a different code boundary CB1(i.e., CB=1), to thereby produce a plurality of samples corresponding tothe code boundary CB1. For example, the sixteen frames sampled bycorrelator 726 b are designated Ratchet Ramp 1 in FIG. 20. Accordingly,correlator 726 b is used to perform step 2102 b, discussed above inconnection with FIGS. 21 and 22.

Concurrently (but beginning slightly later in time), sampling correlator726 c samples a plurality of frames of the received impulse signalpacket based on the ratchet code and a different code boundary CB2(i.e., CB=2), to thereby produce a plurality of samples corresponding tothe code boundary CB2. For example, the sixteen frames sampled at bycorrelator 726 c are designated Ratchet Ramp 2 in FIG. 20. Accordingly,correlator 726 c is used to perform step 2102 c, discussed above inconnection with FIGS. 21 and 22.

Concurrently (but beginning slightly later in time), sampling correlator726 d samples a plurality of frames of the received impulse signalpacket based on the ratchet code and a different code boundary CB3(i.e., CB=3), to thereby produce a plurality of samples corresponding tothe code boundary CB3. The sixteen frames sampled at step 2102 d aredesignated Rachet Ramp 3 in FIG. 20. Accordingly, correlator 726 d isused to perform step 2102 d, discussed above in connection with FIGS. 21and 22.

The outputs S0, S1, S2 and S3 of the correlators are provided tomultiplexers Mux0, Mux1, Mux2 and Mux3. Mux0 selects one of outputs S0,S1, S3 and S3 in accordance with the ratchet code (e.g. P0, P1, P2, P3,P1, P2, P3, P4, P2, P3, P0, P1, P3, P0, P1, P2) and a zeroth codeboundary (CB0). Mux1 selects one of outputs S0, S1, S3 and S3 inaccordance with the ratchet code and a code boundary=1 (CB1). Mux2selects one of outputs S0, S1, S3 and S3 in accordance with the ratchetcode and a code boundary=2 (CB2). Mux3 selects one of outputs S0, S1, S3and S3 in accordance with the short acquisition code (P0, P1, P2, P3)and a code boundary=3 (CB3).

The plurality of samples corresponding to the code boundary CB0 areaccumulated by ramp builder R0 to produce a ratchet ramp value RRV0.Concurrently, the plurality of samples corresponding to the codeboundary CB1 are accumulated by separate ramp builder R1 to produce aratchet ramp value RRV1. Concurrently, the plurality of samplescorresponding to code boundary CB2 are accumulated by ramp builder R2 toproduce a ratchet ramp value RRV2. Concurrently, the plurality ofsamples corresponding to the code boundary CB3 are accumulated by rampbuilder R3 to produce a ratchet ramp value RRV3.

Accordingly multiplexers Mux0, Mux1, Mux2 and Mux3, and ramp buildersR0, R1, R2 and R3 perform steps 2104 a, 2104 b, 2104 c and 2104 d,discussed above in connection with FIG. 21, and also in connection withFIGS. 22 and 23.

As can be appreciated from FIG. 20, Ratchet Ramp 3 is built slightlylater in time than Ratchet Ramp 2, which is built slight later in timethan Ratchet Ramp 1, which is built slightly later in time than RatchetRamp 0. Thus, the earlier generated ratchet ramp values must be storedso that they can be used together with the later generated ratchet rampvalues in a threshold determination. It is noted that additional ratchetramp values (e.g., a second Ratchet Ramp 0) begin to be generated whilelater generated ratchet ramp values (e.g., the first Ratchet Ramp 3) arebeing generated.

Ratchet ramp values RRV0, RRV1, RRV2 and RRV3 are provided to thresholddetector 2408, which determines whether a ratchet threshold (alsoreferred to as a second stage acquisition threshold) has been satisfied.This determination is based on one or more of ratchet ramp values RRV0,RRV1, RRV2 and RRV3. Thus, threshold detector 2408 performs step 2106discussed above in connection with FIGS. 21 and 22. This thresholddetermination may also be based on a back ramp value, which has beendiscussed above, and will be discussed again below. Various methods forperforming this threshold determination, according to the presentinvention, have been discussed in detail above.

If the ratchet threshold has been satisfied, then receiver 2402determines that it has identified the correct code boundary offset, andthus, the impulse radio receiver has achieved code alignment to theextent of the ratchet code.

The threshold may not be satisfied, for example, if the receiver issampling in accordance with the ratchet code when the actual portion ofthe packet header being sampled still includes short acquisition codesequences. Accordingly, if the threshold is not satisfied, additionalframes of the packet header are sampled in search of a ratchet codesequence. If a rachet code sequence is not found within a specificamount of frames (or a specific amount of time), then frame alignmentand/or short code synchronization has most likely been lost. At thatpoint, first stage acquisition starts over again.

The threshold will be satisfied when one of the accumulated ratchet rampvalues consists solely of samples of a ratchet code sequence. Asdiscussed above, if the threshold is satisfied, the ambiguity resultingfrom the integration length being greater than the number of rampbuilders has been resolved to the extent of the ratchet code.Accordingly, if receiver 2402 does not expect any additional ratchetcodes sequences (as defined by the packet protocol), receiver 2402 nowsamples the received signal in an attempt to detect the delimeter (whichis also defined by the packet protocol). If receiver 2402 expectsadditional ratchet code sequences (as defined by the packet protocol),receiver 2402 will sample the received signal in accordance with thenext ratchet code to resolve further ambiguities, as explained above.

As discussed above, an exemplary delimeter is a flipped version of thelongest ratchet code of a packet protocol. Thus, correlators 726 a, 726b, 726 c and 726 d sample the next N frames (e.g., the next 16 frames)based on the longest ratchet code of the protocol, and a multiplexer(e.g., Mux0) and ramp builder (e.g., RO) are used to accumulate theplurality of samples generated in accordance with the longest ratchetcode. The delimeter ramp value produced (e.g. by RO) based on thelongest ratchet code is provided to threshold detector 2408, whichdetermines whether the delimeter threshold has been satisfied. Variousways of performing this threshold determination have been discussed indetail above.

If the delimeter threshold is not satisfied (i.e., the delimeter is notdetected), then receiver 2402 has probably lost frame alignment, andthus, receiver 2402 starts first stage acquisition over again. If thedelimeter threshold is satisfied (i.e., the delimeter is detected), thenreceiver 2402 begins to detect data payload symbols. At this pointsymbol timing has been determined because impulse radio 2402 knowsprecisely when the first frame of the data payload begins, and receiver2402 knows the long code used to encode the symbols in the data payload(e.g., based on the packet protocol).

4. Frame Format and Additional Receiver Embodiments for Use with LongerShort Acquisition Codes and Ratchet Codes

In the exemplary embodiments discussed above, a modulo-4 shortacquisition code was used during first stage acquisition, and fourcorrelators were used to sample each of four possible positions of animpulse within each frame of a header. As noted above, it is likely thatshort acquisition codes of other lengths (e.g., modulo-8 or modulo-16)will be used. In one example, if a modulo-8 short acquisition code isused, then an impulse can be located in one of eight possible positionswithin each frame of a header. Accordingly, eight correlators can beused to sample the eight possible positions of an impulse within eachframe. Alternatively, only four correlators are used to sample the eightpossible positions, in accordance with an embodiment of the presentinvention that shall now be explained with reference to FIG. 26.

Referring now to FIG. 26, a 100 ns frame is shown split into two (2) 50ns subframes. Each of the subframes includes four (4) possiblepositions. More specifically, the 1st subframe includes P0, P1, P2 andP3. The 2nd subframe includes P4, P5, P6 and P7. According to anembodiment of the present invention, each correlator (e.g., 726 a, 726b, 726 c and 726 d) of receiver 2402 is used to sample two (2) possiblepositions during each fame (e.g., each 100 ns frame). This isaccomplished by causing each correlator to sample one position duringeach subframe (e.g., each 50 ns subframe). For example, a firstcorrelator (e.g., 726 a) can sample P0 during the 1st subframe, and thensample P4 during the 2nd subframe. A second correlator (e.g., 726 b) cansample P1 during the 1st subframe, and then sample P5 during the 2ndsubframe. A third correlator (e.g., 726 c) can sample P2 during the 1stsubframe, and then sample P6 during the 2nd subframe. A fourthcorrelator (e.g., 726 d) can sample P3 during the 1st subframe, and thensample P7 during the 2nd subframe. In this manner, the length of theshort codes can be increased without increasing the required number ofcorrelators in receiver 2402. In such an embodiment, there would beeight ramp builders, and the multiplexors would send samples to the rampbuilders in accordance with the modulo-8 short acquisition code.

In another example, a modulo-16 short acquisition code is used, and animpulse can be located in one of sixteen possible positions within eachframe of a header. Accordingly, sixteen correlators can be used tosample the sixteen possible positions of an impulse within each frame.Alternatively, only four correlators are used to sample the sixteenpossible positions, in accordance with an embodiment of the presentinvention that shall now be explained with reference to FIGS. 26 and 27.

Referring first to FIG. 26, shown above each of the eight positions (P0,P1, P2, P3, P4, P5, P6, P7) is a “+” and shown below each position is a“−”. This is meant to represent, that at each position, an impulse canbe a positive impulse or a negative (i.e., a flipped or inverted)impulse. Thus, with eight different positions, and two states at eachposition, a total of sixteen different states can be detected. Ofcourse, the transmitting impulse radio must be able to generate positiveand negative impulses. This has been described in the commonly owned,co-pending U.S. patent application Ser. No. 09/537,692, filed Mar. 29,2000, entitled, “Apparatus, System and Method for Flip Modulation in anImpulse Radio Communication System,” which has been incorporated byreference above.

Still referring FIG. 26, each of the subframes includes four (4)possible positions, wherein an impulse at each position can have twostates (such as “flipped” and “unflipped”, for example). Thus, it is nowmore appropriate to refer to states, rather than simply positions. Forexample, the 1st subframe can include impulses representing states P0,P1, P2, P3, P8, P9, P10 and P11. The 2nd subframe can include impulsesrepresenting states P4, P5, P6, P7, P12, P13, P14 and P15. According toan embodiment of the present invention, each correlator (e.g., 726 a,726 b, 726 c and 726 d) of receiver 2402 is used to sample two (2)possible positions during each fame (e.g., each 100 ns frame). Eachsample is provided to each of sixteen multiplexers. Each sample is alsoinverted and then provided to each of the sixteen multiplexers. In thismanner, the length of the short codes can be further increased withoutincreasing the required number of correlators in a receiver. In such anembodiment, there would be sixteen ramp builders, and the multiplexorswould provide samples to the ramp builders in accordance with themodulo-16 short acquisition code. This is explained in more detail withreference to FIG. 27.

Referring now to FIG. 27, exemplary receiver 2702 includes four samplingcorrelators 726 a, 726 b, 726 c, 726 d. During first stage acquisition:correlator 726 a samples a zeroth impulse position P0 and a fourthimpulse position P4 in accordance with a sampling control signal 736 a;correlator 726 b samples a first impulse position P1 and a fifth impulseposition P5 in accordance with a sampling control signal 736 b;correlator 726 c samples a second impulse position P2 and a sixthimpulse position P6 in accordance with a sampling control signal 736 c;and correlator 726 d samples a third impulse position P3 and a seventhimpulse position P7 in accordance with a sampling control signal 736 d.

A resulting Sample/Hold (S/H) signal 728 a (also referred to as S0, S4),representing correlation results from correlator 726 a, is provided tosixteen multiplexers (Mux0, Mux1 . . . Mux15). Signal 728 a is alsoinverted (the inverted signal is also referred to as S8, S12) andprovided to the sixteen multiplexers.

Similarly, resulting Sample/Hold (S/H) signal 728 b (also referred to asS1, S5), representing correlation results from correlator 726 b, isprovided to the sixteen multiplexers (Mux0, Mux1 . . . Mux15). Signal728 b is also inverted (the inverted signal is also referred to as S9,S13) and provided to the sixteen multiplexers.

Resulting Sample/Hold (S/H) signal 728 c (also referred to as S2, 26),representing correlation results from correlator 726 c, is provided tothe sixteen multiplexers (Mux0, Mux1 . . . Mux15). Signal 728 c is alsoinverted (the inverted signal is also referred to as S10, S14) andprovided to the sixteen multiplexers.

Resulting Sample/Hold (S/H) signal 728 d (also referred to as S3),representing correlation results from correlator 726 d, is provided tothe sixteen multiplexers (Mux0, Mux1 . . . Mux15). Signal 728 d is alsoinverted (the inverted signal is also referred to as S11, S15) andprovided to the sixteen multiplexers.

In accordance with specific embodiments of the present inventiondiscussed above, correlators 726 a, 726 b, 726 c and 726 d sample eachof the possible impulse positions during a frame period (eight possiblepositions in this example) in an attempt to sample impulses of repeatingshort acquisition codes of a packet header. The outputs S0, S1, S2, S3,S4, S5, S6 and S7 of the correlators are provided to sixteenmultiplexers Mux0-Mux15. The outputs S0, S1, S2, S3, S4, S5, S6 and S7are also inverted to produce outputs S8, S9, S10, S11, S12, S13, S14 andS15, which are also provided to the sixteen multiplexers Mux0 throughMux15.

During first stage acquisition, each multiplexer selects one of theoutputs S0-S15 in accordance with the short acquisition code (e.g., P0,P1, P2, P3, P4, P5, P6, P7, P8, P10, P11, P12, P13, P14, P15) and aspecific code offset. For example: Mux0 selects one of outputs S0-S15 inaccordance with the short acquisition code and a zero code offset (CO0),represented by the first column of table 2704; Mux1 selects one ofoutputs S0-S15 in accordance with the short acquisition code and a codeoffset=1 (CO1), represented by the second column of table 2404; and soon.

Mux0 outputs a signal 2706 a, which includes a plurality of samplescorresponding to the code offset CO0 and a frame offset (FO). Thesesamples corresponding to the code offset CO0 and the frame offset (FO)(i.e., signal 2406 a) are provided to a ramp builder R0 (also referredto as an accumulator R0), which accumulates N samples (where N is theintegration length), and outputs a ramp value RV0. Mux1 outputs a signal2706 b, which includes a plurality of samples corresponding to the codeoffset CO1 and the same frame offset (FO). These samples correspondingto the code offset CO1 and the frame offset (FO) (i.e., signal 2406 b)are provided to a ramp builder R1 (also referred to as an accumulatorR1), which accumulates N samples (where N is the integration length),and outputs a ramp value RV1. The remaining multiplexers operate in asimilar fashion such that each of sixteen ramp builders R0-R15 generatesa corresponding ramp value RV0-RV15. These ramp values are provided to athreshold detector, and receiver 2702 operates in a similar manner toreceiver 2402, discussed above in connection with FIG. 24. The majordifference between the operation of receiver 2702 and receiver 2402 isthat receiver 2702 is capable of detecting sixteen different states perframe during acquisition, where the simpler receiver 2402 detects fourdifferent states per frame during acquisition.

If the integration length N is greater than the number of ramp builders(16 in this example), then ratchet codes are used to resolve ambiguitiesrelating to the end of the packet header (which is the same as thebeginning of the data payload). FIG. 23, discussed above, was used toillustrate how ratchet codes can be generated based on a length 4 shortacquisition code. FIG. 28 illustrates how ratchet codes can be generatedbased on a length 16 short acquisition code. The left most column 2802of table 2800 represents a short acquisition code stored, for example,in an acquisition root register of an impulse radio transceiver. Theadditional columns show how longer ratchet codes can be derived by“hopping” or “skipping” positions of the short acquisition code. Theseadditional columns need not be stored in the impulse radio receiver.Rather, an acquisition sequence generator of the impulse radio receivercan generate the portion of a code, shown in the additional columns,based on the short acquisition code stored in the register (i.e., column2802), in the same manner explained above in connection with FIG. 23.During first stage acquisition, an acquisition sequence generator of theimpulse radio transceiver can simply cycle through the short acquisitioncode (e.g., stored in the acquisition root register, and shown at theleft most column 2802 of table 2800) from top to bottom going throughthe length 16 code. To generate longer codes (e.g., ratchet codes) basedon the modulo-16 short acquisition code, the logic (e.g., of theacquisition sequence generator) goes through the left column once butinstead of staring over at 0 (i.e., the zeroth code position) after theend of the first column, the logic “hops” or “skips” one code position.This causes the sequence to hop over code position 0 and start at codeposition 1. The second column from the left shows the sequence after onehop. Thus, if the short code is P0, P1, P2, P3 . . . P15 (as shown inthe left most column 2802), then a length 256 ratchet code can begenerated as shown in the first sixteen columns of table 2800.

Codes up to length 256 can be generated using a length 16 short code andsingle hops. For codes longer than 256, two hops are required to breakup the sequence. For example, to generate a length 512 code, the logicstarts at the upper left corner of table 2300, and single hops at theend of each column until the end of the sixteenth column. At that point,two positions are hopped to produce the seventeenth column. Then oneposition is hopped to produce the eighteenth through thirty firstcolumns. For example, a length 512 ratchet code can obtained using allthirty columns shown in table 2800. To generate even longer codes doublehops are performed at each length 256 code boundary.

When discussing possible states within a frame (wherein a state relatesto where an impulse is located in a frame, and whether it is flipped ornot flipped, e.g., due to a code), the nomenclature Pn (e.g., P0, P1,P2, P3 . . . P15) has been used to represent the different possible(e.g., 16) states. It is noted, however, that state P0, need not beassociated with a position that is earlier in a frame (i.e., closer tothe beginning frame boundary) than states P1, P2 . . . P15. Rather,references to states P0, P1, P2 . . . P15 simply refer to sixteendifferent possible states within a frame.

To summarize the above discussion, 16 states can be sampled using fourcorrelators of a receiver. The receiver samples 8 pulse positions in aframe by firing each of the four correlators twice per frame. Each pulsemay or may not be flipped. The receiver acquisition hardware invertsthese eight samples and routes both the inverted and non-invertedsamples (e.g., using multiplexors) to 16 ramp builders. Thus, fourcorrelators can be used to acquire a packet including modulo-16 shortacquisition code sequences in the header, in accordance with the abovediscussed embodiment of the present invention. More generally, Xcorrelators can be used to acquire a packet including a modulo-(X·4)short acquisition code sequences in the header. One of ordinary skill nthe art will appreciate how the above description enables embodimentswhere different numbers of correlators are used, different code lengthsare used, and the like.

5. IJ or IQ Correlator Pairs and Ramp Builder Pairs

As discussed above in connection with FIGS. 9, 10A, 10B and 10C, someimpulse radios include correlator pairs to produce In-phase (I) andQuadrature (Q) sample pairs (or more generally, I-J sample pairs). Asexplained above, in such embodiments, a Q or J sample is produced at atime slightly delayed from each I sample using a correlator pair. Suchcorrelator pairs can be used in the present invention, as shall beexplained below with reference to FIG. 30.

Referring to FIG. 30, the portion of exemplary receiver 3002 shownincludes I (In-phase) sampling correlators 726 a, 726 b, etc., andcorresponding J or Q sampling correlators 727 a, 727 b, etc. Duringfirst stage acquisition: I correlator 726 a samples a zeroth impulseposition P0 (within a sample frame) in accordance with sampling controlsignal 736 a, and J or Q correlator 727 a samples in accordance with adelayed sampling control signal 3005 a produced by delay 3004 a; Icorrelator 726 b samples a first impulse position P1 in accordance withsampling control signal 736 b, and J or Q correlator 727 b samples inaccordance with a delayed sampling control signal 3005 b produced bydelay 3004 b. Not shown, additional correlator pairs sample a thirdimpulse position P3 (and at a delayed offset) and a fourth impulseposition P4 (and at a delayed offset).

A resulting Sample/Hold (S/H) signal 728 a (also referred to as S0(I)),representing correlation results from correlator 726 a, is provided tofour multiplexers (Mux0(I), Mux1(I), Mux2(I) and Mux4(I)). S/H signal729 a (also referred to as S0(J)), representing the results fromcorrelator 727 a, is provided to four multiplexers (Mux0(J), Mux1(J),Mux2(J) and Mux4(J)). Mux2(I), Mux3(I), Mux2(J) and Mux3(J) are notexplicitly shown in FIG. 30.

Similarly, a resulting Sample/Hold (S/H) signal 728 b (also referred toas S1(I)), representing correlation results from correlator 726 b, isprovided to four multiplexers (Mux0(I), Mux1(I), Mux2(I) and Mux4(I)).S/H signal 729 b (also referred to as S1(J)), representing the resultsfrom correlator 727 b, is provided to four multiplexers (Mux0(J),Mux1(J), Mux2(J) and Mux4(J)).

A resulting Sample/Hold (S/H) signal 728 c (also referred to as S2(I)),representing correlation results from correlator 726 c, is provided tofour multiplexers (Mux0(I), Mux1(I), Mux2(I) and Mux4(I)). S/H signal729 c (also referred to as S2(J)), representing the results fromcorrelator 727 c, is provided to four multiplexers (Mux0(J), Mux1(J),Mux2(J) and Mux4(J)). Correlators 726 c and 727 c are not explicitlyshown in FIG. 30.

A resulting Sample/Hold (S/H) signal 728 d (also referred to as S3(I)),representing correlation results from correlator 726 d, is provided tofour multiplexers (Mux0(I), Mux1(I), Mux2(I) and Mux4(I)). S/H signal729 d (also referred to as S3(J)), representing the results fromcorrelator 727 d, is provided to four multiplexers (Mux0(J), Mux1(J),Mux2(J) and Mux4(J)). Correlators 726 d and 727 d are not explicitlyshown in FIG. 30.

In accordance with specific embodiments of the present inventiondiscussed above, correlators 726 a, 726 b, 726 c and 726 d sample eachof the possible impulse positions during a frame period (four possiblepositions in this example) in an attempt to sample impulses of repeatingshort acquisition codes of a packet header. The outputs S0(I), S1(I),S2(I) and S3(I) of the correlators are provided to four multiplexersMux0(I)-Mux3(I). Correlators 727 a, 727 b, 727 c and 727 d sample attimes slightly delayed (e.g., 0.125 nsec) from the sample times ofcorrelators 726 a, 726 b, 726 c and 726 d, respectively. The outputsS0(J), S1(J), S2(J) and S3(J) of the correlators are provided to fourmultiplexers Mux0(J)-Mux3(J).

During first stage acquisition, each multiplexer Mux0(I)-Mux3(I) selectsone of the outputs S0(I)-S3(I) in accordance with the short acquisitioncode (e.g., P0, P1, P2, P3) and a specific code offset. Similarly, eachmultiplexer Mux0(J)-Mux3(J) selects one of outputs S0(J)-S3(J) inaccordance with the short acquisition code (e.g., P0, P1, P2, P3) and aspecific code offset.

Mux0(I) outputs a signal 3006 a, which includes a plurality of samplescorresponding to the code offset CO0 and a frame offset (FO). Thesesamples corresponding to the code offset CO0 and the frame offset (FO)(i.e., signal 3006 a) are provided to a ramp builder R0(I) (alsoreferred to as an accumulator R0(I)), which accumulates N samples (whereN is the integration length), and outputs a ramp value RV0(I). Mux0(J)outputs a signal 3007 a, which includes a plurality of samplescorresponding to the code offset CO0 and the frame offset (FO). Thesesamples corresponding to the code offset CO0 and the frame offset (FO)(i.e., signal 3007 a) are provided to a ramp builder R0(J) (alsoreferred to as an accumulator R0(J)), which accumulates N samples (whereN is the integration length), and outputs a ramp value RV0(J). Rampvalues RV0(I) and RV0(J) are provided to a ramp value combiner 3010 a,which combines RV0(I) and RV0(J) to produce a single ramp value RV0 tobe used by threshold detector 3008. According to one embodiment of thepresent invention, RV0 is the square root of the sum of the squares oframp values RV0(I) and RV0(J). According to another embodiment, RV0 isthe sum of the absolute values of ramp values RV0(I) and RV0(J). One orordinary skill in the art will appreciate that ramp values RV0(I) andRV0(J) can be combined in other manners while still being within thescope of the present invention. In still another embodiment, the largestramp value of the ramp value pair (e.g., RV0(I)/RV0(J)) is selected asthe ramp value (e.g., RV0) that is provided to threshold detector 3008.A benefit of this embodiment is that the chance of a I/J correlator pair(e.g., 726 a/727 a) sampling a zero crossing of a received impulse (andthus not detecting the impulse) is significantly reduced as compared toa single correlator (e.g., 726 a) sampling for an impulse.

The remaining correlator pairs (i.e., 726 b/727 b, 726 c/727 c, 726d/727 d), multiplexor pairs, and ramp builder pairs operate in a similarfashion to produce combined ramp values (i.e., RV1, RV2 and RV3).Threshold detector 3008 can determine whether the first stage thresholdis satisfied, based on the combined ramp values RV0, RV1, RV2 and RV3,as described in detail above.

During second stage acquisition, receiver 3002 operates in a similarmanner as receiver 2402, except ratchet ramp value pairs (e.g.,RRV0(I)/RRV0(J)) are produced, and then combined to produce a singleratchet ramp value (e.g., RRV0).

The correlator pairs are extremely useful during demodulation of thedata payload because they enable detection of many different types ofmodulated signals, for example, as is described in U.S. patentapplication Ser. No. 09/538,519, entitled “Vector Modulation System andMethod for Wideband Impulse Radio Communications,” and U.S. patentapplication Ser. No. 09/537,692, filed Mar. 29, 2000, entitled,“Apparatus, System and Method for Flip Modulation in an Impulse RadioCommunication System,” both of which have been incorporated by referenceabove.

Analog to digital converters (e.g., A/Ds 772 a-d and 773 a-d) are notexplicitly shown in FIG. 30. However, it is likely that A/Ds are placedat the output of each correlator, so that the inputs to the multiplexersare digital signals. Alternatively, such A/Ds can be placed between themultiplexers and the ramp builders, or between the ramp builders andramp value combiners.

N. Radio Command Channel

In an embodiment of the present invention, the packet protocol, which isknown by impulse radios in communications with one another, also definesthe data integration length (i.e., the number of samples of a receivedsignal that are accumulated to make a symbol decision regarding a symbolin the data payload).

In an alternative embodiment of the present invention, a command channelbetween an impulse radio transmitter and an impulse radio receiver canbe established. More specifically, a predetermined number of commandframes are included between the acquisition delimeter and the datapayload. Thus, in this embodiment, the delimiter refers to one or moreframes in a packet that is used to indicate the end of the header andthe beginning of the command frames. Such command frames include radiocommands bits. For example, in one embodiment, the command framesinclude 8 bits to produce a command byte. The command bits (or bytes)can be used to provide control information, such as data integrationlength, modulation type, and the like. Thus, the data integration lengthneed not be predetermined by the packet protocol. Rather, in thisembodiment of the present invention, the packet protocol can specifyspecific frames (or bits) that define the data integration lengthassociated with the data payload. This would enable impulse radios toadjust (on the fly) the data integration length such that an acceptablesignal to noise ratio (or bit error rate, or the like) is achieved,while still trying to maximize the data throughput.

O. Jittering Impulse Positions

Typically, a relatively long PN code (e.g., code length 512 or greater)is used to break down frequency spectrum comb line structures and spreadthe impulse energy more uniformly across frequencies. Thus, the use ofrelatively short (e.g., length 16 or 32) repeating short acquisitioncode sequences may cause some unwanted frequency spectrum comb lines.Accordingly, an embodiment of the present invention has been developedto spread energy in the frequency domain when short acquisition codesequences of the present invention are transmitted.

According to this embodiment of the present invention, each of thepossible impulse positions is purposely jittered by an impulse radiotransmitter. For example, assume a frame width is 100 ns, and 8 possibleimpulse positions exist within the frame during first stage acquisition.Each of the 8 different positions is likely to be about 10 ns apart fromthe closest other possible positions. For example, referring to FIG. 29,assume that a zeroth possible position (P0) of a frame 2900 is 10 nsfrom the beginning frame boundary 2902, and the first possible position(P1) is 20 ns from the beginning frame boundary 2902. According to thisembodiment of the present invention, each of these possible positions ispurposely jittered approximately 30-40 picoseconds, to help spreadimpulse energy in the frequency domain.

This deliberate jitter can be accomplished in many ways. For example,referring back to FIG. 7, a delay line and a random delay generator(e.g., that together produce a random delay between 30 and 40picoseconds) can be placed between PTG 740 and pulser 608.Alternatively, PTG 740 can be adapted to produce the purposeful ordeliberate jitter. One of ordinary skill in the art will appreciate thatsuch deliberate jitter can be accomplished in many other ways whilestill being within the scope of the present invention. It is noted thatan impulse radio will normally generate about 3-10 picoseconds of jitterwithout trying.

Such jitter (e.g., 30-40 ps) can effectively spread energy in thefrequency domain, while still allowing an impulse radio receiver toachieve first stage acquisition. That is, even though an impulse radioreceiver does not have knowledge of a random jitter pattern, the jitteris sufficiently small that the correlators of the receiver will stillsample energy of receive impulses (e.g., because the width of eachimpulse is approximately 500 ps).

P. Acquisition Time

The present invention enables fast and efficient acquisition of asignal. The following is an exemplary comparison of acquisition usingthe present invention as compared to a typical conventional system.

In this example, assume the following are used with the presentinvention:

-   -   short acquisition code length=16;    -   ramp builders available=8;    -   integration length=256;    -   search step size=3 nsec; and    -   frame size=100 nsec.

Using the present invention, the time required for complete signalacquisition=first stage acquisition time+tracking time+second stageacquisition time. Each of these are explained separately below.

1. First Stage Acquisition Time

First stage acquisition time=(ramp time)·round up (frame length/stepsize)·(code length/ramp builders avail.)=(100 nsec·256)·round up (100nsec/3 nsec)·(16/8)=25,600 nsec·34·2=1,740,800 nsec=1,740.8 microsec.

2. Tracking Time

Tracking time=(# of ramps budgeted for tracking)·(ramp time). Assumethat approximately 25 ramp times (i.e., full ramp integrations) arebudgeted for tracking. Thus, tracking time=25·(100 nsec·256)=640,000nsec=640 microsec.

3. Second Stage Acquisition Time

Second stage acquisition time=(number of integrations)·(ramp time). Thenumber of integrations, and thus second stage acquisition time, isdependent on the number of bits of ambiguity that must be resolvedduring second stage acquisition.

In this example, the total number of bits of ambiguity that must beresolved during signal acquisition (first and second stage together)=log2(integration length)=log 2(256)=8 bits. The number of bits of ambiguityresolved during first stage acquisition=log 2(short acquisition codelength)=log 2(16)=4. Thus, the number of bits of ambiguity that must beresolved during second stage acquisition is equal to the number of bitsthat were not resolved during first stage acquisition=log 2(integrationlength)−log 2(short acquisition code length)=8−4=4 bits that must beresolved during second stage acquisition.

${{The}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {unique}\mspace{14mu} {ratchet}\mspace{14mu} {codes}\mspace{14mu} {required}\mspace{14mu} {to}\mspace{14mu} {resolve}\mspace{14mu} {the}\mspace{14mu} 4\mspace{14mu} {bits}\mspace{14mu} {of}\mspace{14mu} {ambiguity}\mspace{14mu} {that}\mspace{14mu} {must}\mspace{14mu} {be}\mspace{14mu} {resolved}\mspace{14mu} {during}\mspace{14mu} {second}\mspace{14mu} {stage}\mspace{14mu} {acquisition}} = {\left( {{rounded}\mspace{14mu} {up}\mspace{14mu} {to}\mspace{14mu} {next}\mspace{14mu} {integer}} \right){\left\{ \frac{{\log_{2}\left( {\text{int}.{\_ length}} \right)} - {\log_{2}\left( {{{short\_ aqu}.{\_ code}}{\_ length}} \right)}}{\log_{2}\left( {\# {\_ of}{\_ available}{\_ ramp}{\_ builders}} \right)} \right\}.}}$

-   -   The numerator of the above equation is equal to the number of        bits that must be resolved during second stage acquisition. The        denominator is log 2 of the number of ramp builders available        (i.e., used) for signal acquisition during first stage        acquisition, and during second stage acquisition (this does not        include any back ramp builders). Thus, in this example, the        number of unique ratchet codes=rounded up{4/log 2(8)}=rounded        up{4/3}=2 unique ratchet codes. The number of integrations        required during second stage acquisition is thus 2 integrations        for the first (shorter) unique ratchet code, and one integration        for the second (longer) unique ratchet code, equaling a total of        3 integrations.

Thus, the second stage acquisition time=(ramp time)·(number ofintegrations)=(256·100 nsec)·(3)=25,600 nsec·3=76,800 nsec=76.8microsec.

4. Time Required for Complete Acquisition

As mentioned above, time required for complete signal acquisition=firststage acquisition time+tracking time+second stage acquisitiontime=1,740.8 microsec+640 microsec+76.8 microsec=2457.6 microsec≈2,500microsec≈2.5 millisec for complete acquisition in this example. Thisshall now be compared to acquisition using a typical conventionalsystem. If tracking is performed in parallel with second stageacquisition, the complete acquisition time can be further reduced.

5. Time Required in a Typical Conventional System

Assuming a conventional system typically includes one (1) integrator foruse during acquisition (as opposed to 8 parallel ramp builders as in thepresent invention), the time required for conventional signalacquisition=(integration time)·rounded up (frame size/step size)·(lengthof code used for signal acquisition).

During signal acquisition, a convention system uses a much longer codelength than the short acquisition code lengths used in the presentinvention. For this example a conservative conventional code length 512is assumed (however, longer conventional code lengths are typicallyused).

Also assuming, as was the case above:

-   -   integration length=256;    -   search step size=3 nsec; and    -   frame size=100 nsec.

The time required for conventional signal acquisition=(integrationtime)·rounded up (frame size/step size)·(code length)=(256·100nsec)·(100 nsec/3 nsec)·(512)=(25,600 nsec)·(34)·512=445,644,800 nsec445,644.8 microsec≈445,600 microsec≈445.6 millisec.

6. Comparison

A reduction in acquisition time from 445.6 millisec (conventional) to2.5 millisec (present invention) is extremely significant. Thesignificant time reduction is due to the use of the short acquisitioncodes, the ratchet codes, and the parallel ramp builders, all of thepresent invention. Of course, the exact acquisition time is dependent onthe above stated starting assumptions. Also, the above calculations arefor worst case scenarios. For example, in the present invention it maynot be necessary to step through an entire 100 nsec frame, 3 nsec at atime, 34 different times. Rather, first stage acquisition may beaccomplished much earlier. One of ordinary skill in the art willappreciate that even if the above assumptions are varied, the presentinvention will still provide extremely significant reductions inacquisition time.

Q. Hardware and Software Implementations

Specific features of the present invention are performed usingcontrollers. For example, control subsystem 612 (including basebandprocessor 620 and system controller 622) can be implemented ascontrollers. These controllers in effect comprise computer systems orportions of computer systems (e.g., within an impulse radio). Therefore,the following description of a general purpose computer system isprovided for completeness. The present invention can be implemented inhardware, or as a combination of software and hardware. Consequently,the invention may be implemented in the environment of a computer systemor other processing system (e.g., within an impulse radio). An exampleof such a computer system 3100 is shown in FIG. 31. In the presentinvention, all of the received signal processing functions occurringafter received RF signals are down-converted to digitized baseband, canexecute on one or more distinct computer systems 3100. The computersystem 3100 includes one or more processors, such as processor 3104. Theprocessor 3104 is connected to a communication infrastructure 3106 (forexample, a bus or network). Various software implementations aredescribed in terms of this exemplary computer system. After reading thisdescription, it will become apparent to a person skilled in the relevantart how to implement the invention using other computer systems and/orcomputer architectures.

Computer system 3100 also includes a main memory 3108, preferably randomaccess memory (RAM), and may also include a secondary memory 3110. Thesecondary memory 3110 may include, for example, a hard disk drive 3112and/or a removable storage drive 3114, representing a floppy disk drive,a magnetic tape drive, an optical disk drive, etc. The removable storagedrive 3114 reads from and/or writes to a removable storage unit 3118 ina well known manner. Removable storage unit 3118, represents a floppydisk, magnetic tape, optical disk, etc. which is read by and written toby removable storage drive 3114. As will be appreciated, the removablestorage unit 3118 includes a computer usable storage medium havingstored therein computer software and/or data.

In alternative implementations, secondary memory 3110 may include othersimilar means for allowing computer programs or other instructions to beloaded into computer system 3100. Such means may include, for example, aremovable storage unit 3122 and an interface 3120. Examples of suchmeans may include a program cartridge and cartridge interface (such asthat found in video game devices), a removable memory chip (such as anEPROM, or PROM) and associated socket, and other removable storage units3122 and interfaces 3120 which allow software and data to be transferredfrom the removable storage unit 3122 to computer system 3100.

Computer system 3100 may also include a communications interface 3124.Communications interface 3124 allows software and data to be transferredbetween computer system 3100 and external devices. Examples ofcommunications interface 3124 may include a modem, a network interface(such as an Ethernet card), a communications port, a PCMCIA slot andcard, etc. Software and data transferred via communications interface3124 are in the form of signals 3128 which may be electronic,electromagnetic, optical or other signals capable of being received bycommunications interface 3124. These signals 3128 are provided tocommunications interface 3124 via a communications path 3126.Communications path 3126 carries signals 3128 and may be implementedusing wire or cable, fiber optics, a phone line, a cellular phone link,an RF link and other communications channels.

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as removablestorage drive 3114, a hard disk installed in hard disk drive 3112, andsignals 3128. These computer program products are means for providingsoftware to computer system 3100.

Computer programs (also called computer control logic) are stored inmain memory 3108 and/or secondary memory 3110. Computer programs mayalso be received via communications interface 3124. Such computerprograms, when executed, enable the computer system 3100 to implementthe present invention as discussed herein. In particular, the computerprograms, when executed, enable the processor 3104 to implement theprocesses of the present invention, such as methods 1500 (including thesteps explained in connection with FIGS. 15, 16, 21 and 22), forexample. Accordingly, such computer programs represent controllers ofthe computer system 3100. By way of example, in the preferredembodiments of the invention, the processes performed byprocessors/controllers 620 and 622 can be performed by computer controllogic. Also, information necessary for implementation of such processes,such as interference signal predicted frequencies, and so on, are storedin memory 3108 and/or memories 3110 (corresponding to, for example,memory 766). Where the invention is implemented using software, thesoftware may be stored in a computer program product and loaded intocomputer system 3100 using removable storage drive 3114, hard drive 3112or communications interface 3124.

In another embodiment, features of the invention are implementedprimarily in hardware using, for example, hardware components such asApplication Specific Integrated Circuits (ASICs) and gate arrays.Implementation of a hardware state machine so as to perform thefunctions described herein will also be apparent to persons skilled inthe relevant art(s).

R. Automatic Acquisition Threshold

One embodiment of the present invention includes an automaticacquisition threshold setting capability. Whereas the threshold may beset to a predefined value as already described in detail in thisdisclosure, further improvement in performance may be obtained byadjusting the threshold setting based on the signals being received orthe environment. Such adjustment may result in faster acquisition time,more reliable acquisition, and/or a better likelihood of achieving anoptimum lock point. If the threshold is too high, packets may be missed,requiring retransmission and slowing the data throughput. If thethreshold is too low, the system may lock on a weak multipath reflectionresulting in a high bit error rate (BER) or fragile lock, potentiallyrequiring packet retransmission. Thus, establishing an optimum thresholdis essential for best system performance.

The optimum threshold, however, is an elusive quantity. The optimumthreshold value depends on the signal strength, the multipathenvironment, the interference, and potentially, on constructionvariations from system to system. Since the optimum threshold valuedepends on changing system and environment variables, it is difficult toestablish a universal fixed value that is optimum for all conditions.

In one embodiment of the invention, the issue of the variability of theoptimum threshold value is solved by adaptively varying the thresholdvalue in response to system performance measurements.

One such performance measurement, which is available to a system ornetwork with a known or approximately known packet transmission rate, isthe packet loss rate, or the complement, the packet receive rate.

For example, in a network where a hub sends monitor packets at a regularinterval and sends data traffic packets between monitor packets, themonitor packets may be used to establish thresholds so that when dataarrives for a given client, the thresholds are already established andoptimized. Thus, if monitor packets are sent once each millisecond, 1000monitor packets will be sent each second and packet loss rate may bedetermined as the difference between the packets received and packetstransmitted over an interval of time. For example, if 800 packets arereceived over one second, then the packet receive rate is 800 packetsper second and the packet loss rate is 200 packets per second.

It is often beneficial to begin the threshold at a high level and lowerthe threshold into the peaks of the signals being received rather thanstarting at a low value and raising the threshold out of the noise.Beginning at a high level thus assures a greater probability ofacquiring strong signals rather than weak reflections.

Thus, an exemplary process may begin by setting the threshold value at avalue higher than nominal to favor high signal levels. Monitor packetsare then received over an interval of time and the packet loss rate isdetermined. If less than, for example, 50% of the expected monitorpackets are observed, the threshold may be lowered. If over thefollowing interval more monitor packets are observed, the threshold maybe lowered further. The process may be continued until, at some point,further lowering of the threshold may result in fewer packets beingreceived. At such point that fewer packets are received, the thresholdmay be raised again. The threshold may then be alternately slightlyraised and lowered from interval to interval in accordance with packetsreceived (or lost) to maintain optimum performance.

When the threshold is first started at a high level, few, if any,packets may be received and the threshold may be lowered rapidly untilthe number of packets is close to the expected number, at which pointthe threshold may be adjusted at a slower rate. In one such embodiment,the adjustment size is proportional to the packet loss rate and in adirection in accordance with the change in packet loss rate frominterval to interval. In another embodiment, the adjustment size isproportional to the square of the packet loss rate.

Tracking on packets received or packets lost requires that the systemtrack the peak or valley respectively. That is, the error signal,packets lost, has no direction information by which to adjust thethreshold. As described, direction information is derived by whethersuccessive intervals improve performance. Alternatively, the number ofpackets lost may be stored as a function of threshold value and the peakor valley of the function estimated by curve fitting. The minimum ofpackets lost is thus the best value for the threshold. It may bebeneficial to add a dither to the process to insure that values offcenter influence the result. Statistical estimation techniques such asKalman or Bayesian filters may be used to estimate the peak or valley ofthe function. Other methods of tracking a peak may be employed as areknown in the art.

When packets are being received, the threshold may also track theaverage amplitude of the maximum ramp being received. Thus, if thetransmitter is moving or if the path is occluded by a moving object orperson, the threshold will better track the variations in amplitude. Inone such embodiment, the threshold value is the product of a term basedon packet loss rate and a term based on maximum ramp average amplitude.In another embodiment, the threshold is the sum of a term of a termbased on packet loss rate and a term based on maximum ramp averageamplitude.

In one embodiment, each packet contains packet sequence information,such as a packet sequence number, that may be used to determineprecisely the number of lost packets. Packet sequence information isparticulary beneficial in systems where the packet rate is variable orunknown. For example, packets are observed for three intervals, thefirst interval includes packets up to sequence number 323, the secondinterval includes packets with sequence numbers: 324, 325, 327, 330, and331 the third interval begins with a packet sequence number 332. Thusthe missing packets are 326, 328, and 329—three of eight packets missingin the second interval. For this example, an eight packet interval isillustrated; however, typically a larger sample size is used toestablish a more statistically stable average, especially for refiningthe threshold value. Initially the interval may be smaller to effect afaster initialization. Once signals are being acquired with nominalmissing packets, the interval may be lengthened to improve the precisionof the measurement of the missing packet fraction and thus improve theprecision of the threshold setting. Alternatively, the thresholdadjustment size may be lowered to, in effect, use the threshold value toaverage the intervals in an integrating control loop sense.

In an alternative embodiment, frame offset space is scanned to determinethe maximum signal strength and thresholds are initially set below themaximum signal strength. Thresholds may then be further optimized basedon performance criteria.

In a further alternative, frame offset space is scanned to determine anoise floor and a maximum signal strength wherein the threshold value isinitially established at some value, for example half way, between thenoise floor and the maximum signal strength. Thresholds may then befurther optimized based on performance criteria.

In a further embodiment, thresholds may be optimized by a firsttransceiver and the optimization information sent to a secondtransceiver to aid in the reception of the first transceiver by thesecond transceiver. For example, on a network where a hub sends regularcontrol packets that a client may use for optimizing reception, theclient's optimization information may be useful to the hub in receivingdata from the client. This is because the distance and multipath will besymmetrical between the hub and the client, potentially resulting insimilar threshold optimization. The hub must first receive the clientusing a nominal threshold level. Upon receiving the client'sthresholding information, the nominal threshold may be replaced by ormodified by the clients threshold information. Subsequently, the hub mayindependently further optimize the threshold levels.

The threshold adjustment may be applied to any of the thresholdsdescribed in this application or to other architectures using one ormore correlators or to time slot networks or other situations where theknown transmission rate is available.

S. Software Gain Control

In one embodiment, the signal compared with the threshold is the maximumramp value. Since the ramp value is proportional to signal strength, itis beneficial to adjust the threshold value relative to the ramp value.For the same reasons described earlier, if the threshold is above theramp peak, the system will not acquire the ramp. Also, if the thresholdis too low, the system may acquire on a weak reflection of the signal,resulting in poor performance. Thus it is desirable to adjust thethreshold to an optimum point relative to the ramp maximum value.

Depending on the implementation details of the system, the thresholdvalue or threshold comparison may be performed in digital hardware,software, or analog circuits or a combination.

In a further exemplary embodiment, the threshold value may remain fixedand the gain of a variable gain stage before the comparator may bevaried. Varying the gain may be desirable for a number of reasons. Thesignal strength may vary over a wide dynamic range from the closest tothe farthest distance to the transmitter. Accommodating such widedynamic range may require an expensive and power hungry A/D converterfor the digital radio and presents challenges for comparator offsets inan analog radio. These problems may be overcome by varying the gainahead of the comparator, typically using a variable gain RF stage orvariable RF attenuator 706.

FIG. 32 shows a gain plot for a variable gain system wherein both gainand threshold are varied in a coordinated manner, in accordance with thepresent invention. Referring to FIG. 32, the graph 3202 shows gain 3204vs. signal strength on the same graph as threshold 3206 vs. signalstrength. The gain 3204 axis is on the left and the threshold 3206 axisis on the right. As shown, for low signal strength 3208, the gain 3204is varied to maintain a constant ramp maximum value and the threshold3206 is held constant. For high signal strength 3210, the gain 3204 isheld constant and the threshold 3206 is varied for optimum performance.The transition 3212 from high gain to low gain may be based on the gaincontrol value used to control the gain control stage 706; i.e., for lowgain, the gain control is adjusted using closed loop feedback to keep aconstant ramp magnitude, but as the gain control value reaches a limitvalue 3212, the gain control is held constant and the threshold israised to maintain a constant relationship between the threshold and theramp magnitude. An exemplary constant relationship is to keep thethreshold at 50% of the ramp magnitude, or other percentage value as maybe determined by simulation or test. Also, more complex relationshipsmay be used, as have been described in this disclosure.

In an alternative embodiment, the gain 3204 and threshold 3206 may beset or adjusted according to performance measurements such as packetloss rate as described above.

In a further embodiment, the threshold is varied to track signalamplitude for low signal levels below the gain range of the variablegain stage, i.e., signal levels below the level where the maximum gaincan maintain the constant ramp magnitude.

FIG. 33 is a functional flow diagram for a receiver employing a variablegain and threshold system in accordance with the present invention.Referring to FIG. 33, a signal is received by an antenna 602 and coupledto a variable gain stage, which is part of the receiver front end 604.The amplified signal is detected using a correlator 606, or sampler 606or other detector to generate a detected signal. The detection mayinvolve multiple parallel correlators FIG. 7, 727 a-727 d, and 726 a-726d, or other architectures as have been disclosed for UWB systems. Thefollowing functions 3302-3310 may be performed by a baseband processor620 as in FIG. 6, 620. Detection is followed by a summation stage 3302where one or more samples are summed to generate a summation result. Theprogressive summation of samples is also referred to as building ramps.The final result of building ramps is the summation result. Thesummation may involve one sample to tens of thousands or more. Thesummation process may also involve parallel concurrent ramp builders aspreviously described in this disclosure. Further details on parallelramp builders may be found in “SYSTEM AND METHOD FOR PROCESSING SIGNALSIN UWB COMMUNICATIONS”, Ser. No. 10/712,269, filed on 14 Nov. 2003 byBrethour et al, which has been incorporated herein by reference.

The summation process is followed by an evaluation stage 3304 whereevaluation criteria are applied to the ramp value or set of ramp valuesto generate an evaluation result. In one embodiment, the evaluationresult is the summation result. In another embodiment, a group of rampvalues are evaluated using the equations of Table 2A or Table 2B or theother threshold relations disclosed herein. The equations of Table 2Bare substantially equivalent to the equations Table 2A except for therelatively academic difference of replacing a “>” with a “≧”. The groupof ramp values may be generated in sequence by a single correlatorreceiver, or alternatively, the group of ramp values may be generatedconcurrently using parallel ramp builders as previously described inthis disclosure. Other architectures may be used to generate a set oframps for use with evaluation criteria such as the evaluation criteriaof Table 2A or Table 2B. It may be appreciated by one skilled in the artthat additional evaluation criteria may be used within the spirit andteaching of the present invention. It may also be appreciated thatvarying the threshold is equivalent to varying the evaluation criteria,i.e., for any variation in the threshold compared with a givenevaluation criteria, an equivalent variation in the evaluation criteriamay be found to compare with the original threshold. Thus variation ofthe threshold is intended to include equivalent variation of theevaluation criteria.

The evaluation result is then compared with the threshold 3206. If thethreshold 3206 is satisfied, then processing follows as for a potentialsignal detection. If the threshold is not satisfied, then anotherportion of signal space is sampled i.e., typically the timing system 610is shifted to another frame offset value and further samples arereceived 606 and summed 3302. When the potential signal is detected, thesystem 3300 proceeds to check validity, track the signal, and detectdata as described in this disclosure. Once acquisition is achieved, thedetection 606, summation 3302, evaluation 3304, and thresholding 3306resources are directed by the signal processor 3308 to receive data anddeliver a data output 3312.

The threshold value and gain value may be set in accordance with theprocesses described in this disclosure. For example, in one embodiment,the evaluation criteria is the maximum ramp value and the gain is set tocontrol the maximum ramp value to be a predetermined value within thelimits of the gain control range of the variable gain amplifier 604 orattenuator 706. The threshold is then set to 50% of a long term averageof the maximum ramp value. The threshold value may track the variationsin the maximum ramp value if the signal level exceeds the high or lowlimits of the control range for the gain control stage 604, 706.

In a further embodiment, where multiple transmitters are received inturn by the receiver, the gain and threshold values may be stored inmemory 3310, typically called context memory 3310, and the gain andthreshold values may be read from memory to prepare to receive eachrespective transmitter in turn.

In a further exemplary embodiment, the summation stage comprisesparallel ramp builders and the evaluation criteria includes one of theequations of Table 2A or Table 2B. Some of the equations of Table 2A areratiometric with respect to the amplitude of the signal, that is, thenumerator and denominator of the ratio are both proportional to signalamplitude so that the equation result is insensitive to signalamplitude.

In a further embodiment, multiple criteria, such as the criteria ofTable 2A, may be calculated and the criterion selected based on aperformance measure. Fore example, two or more of the criteria may beused and the associated packet loss rate recorded for each with thebetter performing criteria selected.

In a further embodiment, multiple criteria, such as the criteria ofTable 2A, may be calculated and the criterion selected based on anapplication characteristic. One such application characteristic is avariable link range. For example, applications which require supportinghighly variable link ranges will preferentially use the table entrieswhich are ratiometric.

Gain may be set to establish the signal amplitude properly within thedynamic range of the system. Thus, the gain may be adjusted to deliverthe ramp maximum amplitude at a predetermined value. The threshold valuemay then be adjusted based on performance criteria such as packet lossrate, as has been described herein.

T. Automatic Multi-Stage Threshold Adjustment

FIG. 34 is a functional flow diagram illustrating an exemplarymulti-stage acquisition process in accordance with the presentinvention. A multi stage acquisition process may include one or more ofthe following stages:

Initial Detection—First detection of a possible signal.

Initial Tracking—Initial closed loop tacking of the signal

Sig-Opt—Signal Optimization, refinement of tracking loop lock point

Quick Check—Verification of the first detection.

Lock Check—verification of tracking loop lock

Long Code (Ratchet Code) Detection—Detection of long length code

Detect Delimiter—Detect end of acquisition header

Although, in a preferred embodiment, all of the above listed stages areimplemented, one or more stages may be eliminated to save some cost orcomplexity. Each stage may have a threshold process. Further, thethreshold for each stage may be different and the optimum value maychange differently for different signal conditions. Thus, it isdesirable to modify the respective threshold value separately andindependently for each stage of acquisition. The exemplary multistageacquisition process may be applied to packet acquisition wherein thepacket has a special acquisition header providing a sequence of knowncoded signals tailored for each stage of acquisition.

Referring to FIG. 34, FIG. 34 illustrates a multistep acquisition system3402 performing a number of stages of acquisition 3404-3414, each basedon receiving and processing 3420-3434 a group of signal samples underthe guidance of a controller 3440. The controller 3440 may change gainsettings 3442, code patterns 3444, threshold criteria 3446, thresholdvalues 3448, and monitors and directs the sequence 3450 of acquisitionsteps 3404-3414.

During the first stage of acquisition, initial signal detection 3404, asignal is received and amplified by an adjustable gain stage 3422. Acode is selected 3424. The code may be different for different stages ofdetection. Initial detection 3404 may use a short code, shorter than theintegration length used to generate ramps 3428, to allow faster initialdetection 3404 of the signal. Acquisition on the short code leavesambiguities that are resolved in later stages. Later stages may select alonger code 3424 to resolve the ambiguities.

The signal is then detected 3426 by a correlator or sampler. In oneembodiment, the correlators are provided in pairs, I and J (or Q). The Icorrelator receives the direct signal which is fed to the summation 3428(ramp building) process to detect data or build signal for thresholds.The J correlator is delayed (or advanced) by a time shift of ¼ cycle ofthe center frequency of the UWB signal. Alternatively, a Q correlatormay be based on ¼ wave shifting of the signal. The J or Q correlatorsignal may be combined with the I signal to generate a magnitude signalfor ramp building. Combining the I and J signal may be by square root ofthe sum of the squares, or by summation of absolute values. Typicallythe J or Q signal is also used as the tracking loop 3406 error signaland is filtered and fed to the timing system 610 to maintain trackinglock on the signal.

Once detected, the detected signal may be summed with other samples togenerate ramps 3428. One or more samples may be used to generate a ramp.In first stage acquisition, a ramp may comprise several, for example 4,short code modulos. Ramps may be generated by a single correlator and asingle ramp builder, or multiple correlators and a single ramp builder,or by multiple correlators and multiple ramp builders, as describedearlier in this disclosure.

One or more ramps are used to generate threshold criteria 3430 that arethen compared with a threshold 3432. The threshold criteria in oneembodiment, may be the ramp value. In other embodiments, the thresholdcriteria may be a function of multiple ramp values as has beenpreviously described in this disclosure.

During initial detection, if the threshold criteria are met, this meansthat there is a reasonable probability that the detection represents adesirable signal. Further steps 3408 are then initiated to verify theinitial detection; however, it is desirable to close the tracking loop3406 upon initial detection to better align the correlator with thesignal timing and prevent the signal from drifting away due to frequencydifferences between the receiver time base and the signal. A Sig-Opt3406 process may be performed to scan the neighborhood (signals within asmall frame offset from the detected signal) to see if a higher peak isavailable for reception. If a higher peak is found, the acquisitionprocess is redirected to the frame offset value associated with thehigher peak and the process continues with the next stage ofacquisition.

After initial detection 3404, there still exists a significantprobability that the detection represents a noise spike. Thus, thesystem 3402 then performs a quick check 3408 operation to furthervalidate the initial detection 3404. In one embodiment, a single initialdetection ramp is produced and compared with a quick check threshold3432. In an alternative embodiment, the quick check 3408 operation is asequence of several, for example five, initial detections 3404 whereinfor example, four out of five, of the initial detections 3404 mustsatisfy the quick check threshold 3432 to pass quick check 3408. In afurther alternative, a longer integration 3428 may be used.

In any of the alternatives for quick check 3408, the threshold setting3448 may be different from the threshold setting 3448 for initialdetection 3404. If quick check 3408 is passed, there is a much higherprobability that the detection is a signal and not a noise spike. Afterquick check 3408, a check lock 3410 process may be performed to verifythat the lock point is at least as good as the original detection, i.e.that Sig-Opt did not lose the signal, if a signal were present.Alternatively, check lock may proceed in parallel with quick checkbecause the tracking function may use different system resources(correlators and ramp builders) than the threshold generation function.

Once quick check 3408 is passed, (and check lock, if used), the systemis configured to look for a long code 3412. (The long code is alsoreferred to as the ratchet code in this disclosure.) When the long codeportion of the header is received, reception of the long code 3412 willhelp resolve remaining timing ambiguities resulting from receiving theshort code. To configure for receiving the long code 3412, a portion ofthe system correlator resources 3426 are reconfigured to utilize thelong code. It is desirable that a portion of system resources 3426remain configured to receive the short code to maintain tracking lock3406 on the signal and maintain at least short code synchronization withthe signal. In the case of multiple parallel correlators, some of themultiple parallel correlators may utilize the long code and one or twocorrelators may utilize the short code. While looking for long codes,the correlators utilizing the short code may be a small portion of thetotal system correlator resources and thus may be considered abackground process. Accordingly, such signal summations are also termedback ramps in this disclosure. Alternatively, reception of the long codemay be time division multiplexed with the short code as may be done by asingle correlator or dual correlator receiver.

Since reception of the long code involves distinguishing among codeoffset values that are a function of the short code length, thethresholds may be tailored to distinguish different code offset ramps.Thus, the long code thresholds may be different from the short codethresholds.

The end of the long code portion of the header may be marked by adelimiter 3414. In one embodiment, the delimiter is a long code patternmodulated with the opposite polarity. Thus, the delimiter may bereceived by the correlators configured for the long code. The delimiterdetection process 3414, however, may have a different threshold tailoredto distinguishing between a positive and negative ramp polarity.

If the thresholds for each different stage are out of balance (properrelationship among one another), one stage may be responsible for toomany lost packets. For example, if the first stage 3404 threshold is toohigh, good signals will be rejected. If the first stage 3404 thresholdis too low, too many noise spikes will be passed to quick check 3408,slowing the acquisition process potentially to the point that theheader, with the special lock code, is completed before lock isachieved. If quick check 3408 threshold is too high, good signals willbe rejected as well. If quick check 3408 threshold is too low, lock maybe attempted on noise, or noise may be searched in vain for a delimiter,slowing the acquisition process and missing packets. Thus, it isdesirable to set each threshold optimally and separately.

In one embodiment, packet loss rate is used to adjust the thresholds.Thresholds may be established initially at a nominal level previouslydetermined to work well under typical environments. As packets arereceived, the packet loss rate may be derived as previously described inthis disclosure. One of the thresholds, for example the initialacquisition threshold, may then be varied. A sequence of packets is thenreceived at the higher threshold and the new packet loss rate is noted.If the packet loss rate is improved, then the threshold may be movedagain in the same direction. If the packet loss rate is decreased, thethreshold may be moved back. The process continues until equilibrium isestablished. Once equilibrium is established with one threshold, adifferent threshold is selected for variation. When all thresholds areadjusted, the process may begin again.

It is typically desirable to limit the automatic variation of thresholdsbetween limits previously determined during design and testing of thesystem because extreme conditions may momentarily drive a threshold intoa degraded, but stable operating region.

In an alternate embodiment, each threshold may be selected foradjustment in turn after only one adjustment at a previously selectedthreshold. Thus, all thresholds may be adjusted toward equilibriumrelatively concurrently.

In another embodiment, activity at an intermediate stage may be used toset an earlier threshold. For example, it is desirable to place theinitial acquisition threshold at a low level to avoid missing goodsignals due to a noise spike (that subtracts from the signal). However,if the threshold is too low, there will be too many initial detections,each requiring a quick check operation, slowing the acquisition process.Thus the quick check false alarm rate may be used to adjust the initialdetection threshold, e.g., if the quick check false alarm rate is toohigh, the initial detection threshold may be raised. Alternatively, thetime required to scan one frame offset modulo may be used to adjust theinitial detection threshold, e.g. if the time is too long, the initialdetection threshold may be raised.

In an alternate embodiment, the thresholds may be set according tomeasured environmental characteristics, such as signal strength ormultipath or interference. Signal strength may be indicated by the gainsetting in the gain control loop together with ramp amplitude values.Multipath may be measured by scanning through a modulo of frame offsetsand observing the peaks below the main peak. The thresholds may be setto reject peaks below the main peak, i.e., threshold criteria peaks areobserved as signal space is scanned over a range of frame offset valuesand the threshold is set to receive the strongest signal and reject thenext strongest signal. Interference may be measured by observing peakvalue of a sequence of samples or ramps that are not associated with areceived packet. Thresholds may be set to reject interference peaks.

In a further embodiment, the multipath environment may be measured byscanning using a receiver to scan frame offset space as a transmittertransmits a known signal to be measured. The resulting scan isindicative of the various peak signal values related to the variousmultipath reflections in the path between the transmitter and receiver.Thus, the threshold may be set to receive the strongest signal andreject the next strongest reflection based on a prior scan of themultipath environment. The threshold value may also be set just belowthe peak value based on a measurement of the multipath envelope decayrate. Measurement of multipath signals is further explained in U.S. Pat.No. 6,700,538, titled “System and Method for Estimating SeparationDistance Between Impulse Radios Using Impulse Signal Amplitude,” whichissued Mar. 2, 2004 to Richards.

In a further embodiment of a multistage acquisition system, wheremultiple transmitters are received in turn by the receiver, the gain andthreshold values may be stored in memory, typically called contextmemory, and the gain and threshold values may be read from memory toprepare to receive each respective transmitter in turn.

IV. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like or any combination thereof.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

All cited patent documents and publications in the above description areincorporated herein by reference.

1. An ultra wideband transceiver system comprising: an ultra widebandtransmitter; an ultra wideband antenna coupled to the uwb transmitter;an ultra wideband receiver coupled to the uwb antenna for receiving aninput signal, said ultra wideband receiver further comprising: asampling stage for sampling the input signal and generating a sampledsignal; a summation stage for summing a plurality of sampled signals andgenerating a sum signal; an evaluation stage for evaluating one or moresum signals to generate an evaluation result; a threshold comparator tocompare the evaluation result with a threshold; wherein the threshold isvaried based on a performance criterion.
 2. The system of claim 1wherein the timing is adjusted to receive signals synchronous with adifferent frame offset value and a new set of sampled signals isgenerated and evaluated.
 3. The system of claim 1 further includingmultiple parallel ramp builders to generate a plurality of concurrentlygenerated sum signals wherein said plurality of concurrently generatedsum signals is evaluated to generate the evaluation result.
 4. Thesystem of claim 1 wherein the input signal includes packet sequenceinformation and the receiver utilizes said packet sequence informationfor setting said threshold.
 5. A network of UWB transceivers having aprotocol, said protocol including packet count information, said networkcomprising; at least one transmitter sending a plurality of packets; atleast one receiver receiving a portion of said plurality of packets;wherein said at least one transmitter includes said packet countinformation in at least one packet and said receiver uses said packetcount information to adjust at least one threshold.
 6. The network ofclaim 5 wherein said packet count information is a packet sequencenumber.
 7. An ultra wideband receiver comprising: an ultra widebandantenna; an ultra wideband receiver coupled to the uwb antenna forreceiving an input signal, said ultra wideband receiver furthercomprising: a sampling stage for sampling the input signal andgenerating a sampled signal; a summation stage for summing a pluralityof sampled signals and generating a sum signal; an evaluation stage forevaluating one or more sum signals to generate an evaluation result; athreshold comparator to compare the evaluation result with a threshold;wherein the threshold is varied based on a performance criterion.
 8. Thereceiver of claim 7 wherein the timing is adjusted to receive signalssynchronous with a different frame offset value and a new set of sampledsignals is generated and evaluated.
 9. The receiver of claim 7 whereinthe receiver includes a single correlator.
 10. The receiver of claim 7further including a memory, wherein the receiver receives the signalfrom a given transmitter a plurality of times, the threshold parametersassociated with the given transmitter are stored and retrieved from thememory and utilized to search for the given transmitter.
 11. Thereceiver of claim 7 wherein said receiver receives a plurality ofpackets wherein the number of missed packets can be estimated; whereinsaid receiver adjusts at least one threshold based on said number ofmissed packets
 12. An ultra wideband receiver comprising: an ultrawideband antenna; an ultra wideband receiver coupled to the uwb antennafor receiving an input signal, said ultra wideband receiver furthercomprising: a sampling stage for sampling the input signal andgenerating a sampled signal; a summation stage for summing a pluralityof sampled signals and generating a sum signal; a first evaluation stagefor evaluating one or more sum signals to generate an evaluation result;a threshold comparator to compare the first evaluation result with athreshold; a plurality of subsequent evaluation stages for comparing oneor more sum signals to generate a evaluation results; wherein data isdemodulated after all of the thresholds are satisfied.
 13. The receiverof claim 12 wherein the threshold of the first evaluation stage isvaried based on the success or failure rates of the evaluations in thesubsequent stages.
 14. The receiver of claim 12 wherein a plurality ofthe thresholds are adjusted based on the success rates of one or more ofthe evaluation stages.
 15. The receiver of claim 12 further including amemory, wherein the receiver receives the signal from a giventransmitter a plurality of times, the threshold parameters associatedwith the given transmitter are stored and retrieved from the memory andutilized to search for the given transmitter.
 16. An ultra widebandreceiver comprising: an ultra wideband antenna; an ultra widebandreceiver coupled to the uwb antenna for receiving an input signal, saidultra wideband receiver further comprising: a sampling stage forsampling the input signal and generating a sampled signal; a summationstage for summing a plurality of sampled signals and generating a sumsignal; a gain control loop having a gain setting, said gain controlloop for compressing the dynamic range of the sum signal; an evaluationstage for evaluating one or more sum signals to generate an evaluationresult; a threshold comparator to compare the evaluation result with athreshold; wherein the threshold is adjusted based on said gain setting.wherein data is demodulated after the threshold is satisfied.
 17. Thereceiver of claim 16 wherein gain is varied based on a long term averageof the maximum ramp value.
 18. The receiver of claim 17 wherein the gaincontrol loop has a gain limit and the threshold is varied when the gainlimit is reached.
 19. A method for receiving an ultra wideband signal,said method comprising: sampling said ultra wideband signal to generatea sampled signal; summing a plurality of sampled signals to generate asum signal; generating an evaluation criteria based on at least one saidsum signal; comparing said evaluation criteria with a predeterminedthreshold value; proceeding with further processing when the thresholdis met; generating a performance criterion relating to the thresholdvalue; adjusting the threshold value based on the performance criteria;20. The method of claim 19 wherein the receiving includes a short codereceiving process and the threshold value relates to receiving shortcode signals.
 21. The method of claim 19 wherein the receiving includesa long code receiving process and the threshold value relates toreceiving long code signals.
 22. The method of claim 19 wherein thesampling step includes sampling by multiple correlators.
 23. The methodof claim 19 wherein the summing step includes summing by multipleparallel ramp builders.
 24. The method of claim 19 wherein thegenerating step is based on multiple sum signals.
 25. The method ofclaim 19 wherein the performance criterion is based on packet receivedrate.
 26. The method of claim 19 wherein the performance criterion isbased on a packet loss rate.
 27. A method of acquiring an ultra widebandsignal, comprising the steps of: (a) establishing an acquisitionthreshold; (b) obtaining a template signal; (c) receiving the ultrawideband signal; (d) comparing the received ultra wideband signal andthe template signal to produce a comparison result; (e) determiningwhether the comparison result satisfies the acquisition threshold; and(f) if the comparison result fails to satisfy the acquisition threshold,varying the acquisition threshold based on a performance criteria andreturning to step (b), and if the comparison result satisfies theacquisition threshold, locking on the received ultra wideband signal.28. A method of acquiring an ultra wideband signal, comprising: varyingthe timing of a generated template signal until a comparison of thetemplate signal and the ultra wideband produces a comparison result thatsatisfies an acquisition threshold, wherein the acquisition threshold isvaried in accordance with a performance criteria.
 29. A method ofacquiring an ultra wideband signal, comprising: satisfying each of aplurality of thresholds wherein at least one of said plurality ofthresholds is adjusted based on a performance criteria.